How do I convert a libGDX GWT game to version 1.9.5?

Question

I am trying to upgrade my game to libGDX 1.9.5. The game works fine on desktop, but when I try to build the HTML version I get the following error:

Configuration on demand is an incubating feature.
:core:compileJava UP-TO-DATE
:core:processResources UP-TO-DATE
:core:classes UP-TO-DATE
:core:jar UP-TO-DATE
:html:compileJava UP-TO-DATE
:html:processResources UP-TO-DATE
:html:classes UP-TO-DATE
:html:addSource
:html:compileGwt
Loading inherited module 'tech.otter.merchant.GdxDefinition'
   Loading inherited module 'com.badlogic.gdx.backends.gdx_backends_gwt'
      Loading inherited module 'com.google.gwt.user.User'
         Loading inherited module 'com.google.gwt.media.Media'
            Loading inherited module 'com.google.gwt.user.UI'
               Loading inherited module 'com.google.gwt.uibinder.UiBinder'
                  [ERROR] Line 20: Unexpected element 'resource'
                  [ERROR] Failure while parsing XML
com.google.gwt.core.ext.UnableToCompleteException: (see previous log entries)
        at com.google.gwt.dev.util.xml.DefaultSchema.onUnexpectedElement(DefaultSchema.java:86)

Historically, the biggest challenges for me when using GWT have been VisUI and gdx-kiwi, but I have gone to their wikis and updated their versions in my gradle files shown below:

/build.gradle

buildscript {
    repositories {
        mavenLocal()
        mavenCentral()
        maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
        jcenter()
    }
    dependencies {
        classpath 'de.richsource.gradle.plugins:gwt-gradle-plugin:0.6'
        classpath 'com.android.tools.build:gradle:2.2.0'
        classpath "com.badlogicgames.gdx:gdx-tools:1.9.4"
    }
}

allprojects {
    apply plugin: "eclipse"
    apply plugin: "idea"

    version = '1.1'
    ext {
        appName = "merchant"
        gdxVersion = '1.9.5'
        roboVMVersion = '2.2.0'
        box2DLightsVersion = '1.4'
        ashleyVersion = '1.7.0'
        aiVersion = '1.8.0'
        kiwiVersion = '1.8.1.9.4'
        visVersion = '1.3.0-SNAPSHOT'
    }

    repositories {
        mavenLocal()
        mavenCentral()
        maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
        maven { url "https://oss.sonatype.org/content/repositories/releases/" }
    }
}

project(":desktop") {
    apply plugin: "java"

    dependencies {
        compile project(":core")
        compile "com.badlogicgames.gdx:gdx-backend-lwjgl:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-desktop"
        compile "com.badlogicgames.gdx:gdx-tools:$gdxVersion"
        compile "com.github.czyzby:gdx-kiwi:$kiwiVersion"
    }
}

project(":android") {
    apply plugin: "android"

    configurations { natives }

    dependencies {
        compile project(":core")
        compile "com.badlogicgames.gdx:gdx-backend-android:$gdxVersion"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-armeabi"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-armeabi-v7a"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-arm64-v8a"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-x86"
        natives "com.badlogicgames.gdx:gdx-platform:$gdxVersion:natives-x86_64"
        compile "com.badlogicgames.gdx:gdx-ai:$aiVersion"
    }
}

project(":html") {
    apply plugin: "gwt"
    apply plugin: "war"

    dependencies {
        compile project(":core")
        compile "com.badlogicgames.gdx:gdx-backend-gwt:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx:$gdxVersion:sources"
        compile "com.badlogicgames.gdx:gdx-backend-gwt:$gdxVersion:sources"
        compile "com.badlogicgames.gdx:gdx-ai:$aiVersion:sources"
        compile "com.github.czyzby:gdx-kiwi:$kiwiVersion:sources"
        compile "com.kotcrab.vis:vis-ui:$visVersion:sources"
    }
}

project(":core") {
    apply plugin: "java"

    dependencies {
        compile "com.badlogicgames.gdx:gdx:$gdxVersion"
        compile "com.badlogicgames.gdx:gdx-ai:$aiVersion"
        compile "com.github.czyzby:gdx-kiwi:$kiwiVersion"
        compile "com.kotcrab.vis:vis-ui:$visVersion"
    }
}

tasks.eclipse.doLast {
    delete ".project"
}

html/build.gradle

apply plugin: "java"
apply plugin: "jetty"

gwt {
    gwtVersion='2.6.1' // Should match the gwt version used for building the gwt backend
    maxHeapSize="1G" // Default 256m is not enough for gwt compiler. GWT is HUNGRY
    minHeapSize="1G"

    src = files(file("src/")) // Needs to be in front of "modules" below.
    modules 'tech.otter.merchant.GdxDefinition'
    devModules 'tech.otter.merchant.GdxDefinitionSuperdev'
    project.webAppDirName = 'webapp'

    compiler {
        strict = true;
        enableClosureCompiler = true;
        disableCastChecking = true;
    }
}

task draftRun(type: JettyRunWar) {
    dependsOn draftWar
    dependsOn.remove('war')
    webApp=draftWar.archivePath
    daemon=true
}

task superDev(type: de.richsource.gradle.plugins.gwt.GwtSuperDev) {
    dependsOn draftRun
    doFirst {
        gwt.modules = gwt.devModules
    }
}

task dist(dependsOn: [clean, compileGwt]) {
    doLast {
        file("build/dist").mkdirs()
        copy {
            from "build/gwt/out"
            into "build/dist"
        }
        copy {
            from "webapp"
            into "build/dist"
            }
        copy {
            from "war"
            into "build/dist"
        }
        delete "../../madigan.github.io/merchant/"
        copy {
            from "build/dist"
            into "../../madigan.github.io/merchant/"
        }
        doGit
    }

}

task doGit(type: Exec) {
    commandLine 'git', '-C', '../../madigan.github.io', 'add', '.'
    commandLine 'git', '-C', '../../madigan.github.io', 'commit', '-m', '"Distribution."'
    commandLine 'git', '-C', '../../madigan.github.io', 'push'
}

draftWar {
   from "war"
}

task addSource << {
    sourceSets.main.compileClasspath += files(project(':core').sourceSets.main.allJava.srcDirs)
}

tasks.compileGwt.dependsOn(addSource)
tasks.draftCompileGwt.dependsOn(addSource)

sourceCompatibility = 1.8
sourceSets.main.java.srcDirs = [ "src/" ]


eclipse.project {
    name = appName + "-html"
}

Any advice or tips on how to troubleshoot this error would be much appreciated.

[EDIT]

I read the release notes a little closer, and it says that I should change the GWT version to 2.8.0. Even when I do this, I still get the following error:

Configuration on demand is an incubating feature.
:core:compileJava UP-TO-DATE
:core:processResources UP-TO-DATE
:core:classes UP-TO-DATE
:core:jar UP-TO-DATE
:html:compileJava UP-TO-DATE
:html:processResources UP-TO-DATE
:html:classes UP-TO-DATE
:html:addSource
:html:compileGwt
Unknown argument: -XenableClosureCompiler
Google Web Toolkit 2.8.0
Compiler [-logLevel (ERROR|WARN|INFO|TRACE|DEBUG|SPAM|ALL)] [-workDir dir] [-X[no]closureFormattedOutput] [-[no]compileReport] [-X[no]checkCasts] [-X[no]classMetadata] [-[no]draftCompile] [-[no]checkAssertions] [-XfragmentCount numFragments] [-XfragmentMerge numFragments] [-gen dir] [-[no]generateJsInteropExports] [-XmethodNameDisplayMode (NONE|ONLY_METHOD_NAME|ABBREVIATED|FULL)] [-Xnamespace (NONE|PACKAGE)] [-optimize level] [-[no]saveSource] [-setProperty name=value,value...] [-style (DETAILED|OBFUSCATED|PRETTY)] [-[no]failOnError] [-[no]validateOnly] [-sourceLevel [auto, 1.8]] [-localWorkers count] [-[no]incremental] [-war dir] [-deploy dir] [-extra dir] [-saveSourceOutput dir] module[s] 

where 
  -logLevel                      The level of logging detail: ERROR, WARN, INFO, TRACE, DEBUG, SPAM or ALL (defaults to INFO)
  -workDir                       The compiler's working directory for internal use (must be writeable; defaults to a system temp dir)
  -X[no]closureFormattedOutput   EXPERIMENTAL: Enables Javascript output suitable for post-compilation by Closure Compiler (defaults to OFF)
  -[no]compileReport             Compile a report that tells the "Story of Your Compile". (defaults to OFF)
  -X[no]checkCasts               EXPERIMENTAL: DEPRECATED: use jre.checks.checkLevel instead. (defaults to OFF)
  -X[no]classMetadata            EXPERIMENTAL: Include metadata for some java.lang.Class methods (e.g. getName()). (defaults to ON)
  -[no]draftCompile              Compile quickly with minimal optimizations. (defaults to OFF)
  -[no]checkAssertions           Include assert statements in compiled output. (defaults to OFF)
  -XfragmentCount                EXPERIMENTAL: Limits of number of fragments using a code splitter that merges split points.
  -XfragmentMerge                DEPRECATED (use -XfragmentCount instead): Enables Fragment merging code splitter.
  -gen                           Debugging: causes normally-transient generated types to be saved in the specified directory
  -[no]generateJsInteropExports  Generate exports for JsInterop purposes (defaults to OFF)
  -XmethodNameDisplayMode        EXPERIMENTAL: Specifies method display name mode for chrome devtools: NONE, ONLY_METHOD_NAME, ABBREVIATED or FULL (defaults to NONE)
  -Xnamespace                    Puts most JavaScript globals into namespaces. Default: PACKAGE for -draftCompile, otherwise NONE
  -optimize                      Sets the optimization level used by the compiler.  0=none 9=maximum.
  -[no]saveSource                Enables saving source code needed by debuggers. Also see -debugDir. (defaults to OFF)
  -setProperty                   Set the values of a property in the form of propertyName=value1[,value2...].
  -style                         Script output style: DETAILED, OBFUSCATED or PRETTY (defaults to OBFUSCATED)
  -[no]failOnError               Fail compilation if any input file contains an error. (defaults to ON)
  -[no]validateOnly              Validate all source code, but do not compile. (defaults to OFF)
  -sourceLevel                   Specifies Java source level (defaults to 1.8)
  -localWorkers                  The number of local workers to use when compiling permutations
  -[no]incremental               Compiles faster by reusing data from the previous compile. (defaults to OFF)
  -war                           The directory into which deployable output files will be written (defaults to 'war')
  -deploy                        The directory into which deployable but not servable output files will be written (defaults to 'WEB-INF/deploy' under the -war directory/jar, and may be the same as the -extra directory/jar)
  -extra                         The directory into which extra files, not intended for deployment, will be written
  -saveSourceOutput              Overrides where source files useful to debuggers will be written. Default: saved with extras.
and 
  module[s]                      Specifies the name(s) of the module(s) to compile
:html:compileGwt FAILED

[EDIT #2]

Now that the flag is removed per @Colin Altworth's answer, I get the following error particular to vis-ui (again, updated to use the recommended version for libGDX 1.9.5):

:html:compileGwt
Compiling module tech.otter.merchant.GdxDefinition
   Tracing compile failure path for type 'com.kotcrab.vis.ui.util.async.AsyncTask'
      [ERROR] Errors in 'jar:file:/home/john/.gradle/caches/modules-2/files-2.1/com.kotcrab.vis/vis-ui/1.3.0-SNAPSHOT/e5f8abbdc3524212158f65d63c9a28d226795f97/vis-ui-1.3.0-SNAPSHOT-sources.jar!/com/kotcrab/vis/ui/util/async/AsyncTask.java'
         [ERROR] Line 46: The constructor Thread(new Runnable(){}, String) is undefined
         [ERROR] Line 51: The method start() is undefined for the type Thread
   [ERROR] Aborting compile due to errors in some input files
:html:compileGwt FAILED

FAILURE: Build failed with an exception.

[EDIT #3]

Turns out the rest of the issue was a bug in VisUI- the latest update tries to use the Thread class, which is not supported by GWT.

Answer 1

Unknown argument: -XenableClosureCompiler

Remove the closure flag from your gradle GWT build:

gwt {
    gwtVersion='2.6.1' // Should match the gwt version used for building the gwt backend
    maxHeapSize="1G" // Default 256m is not enough for gwt compiler. GWT is HUNGRY
    minHeapSize="1G"

    src = files(file("src/")) // Needs to be in front of "modules" below.
    modules 'tech.otter.merchant.GdxDefinition'
    devModules 'tech.otter.merchant.GdxDefinitionSuperdev'
    project.webAppDirName = 'webapp'

    compiler {
        strict = true;
        enableClosureCompiler = true;//<------------------------REMOVE THIS
        disableCastChecking = true;
    }
}

GWT no longer supports this experimental flag.

Answer 2

Html module of libGDX version 1.9.5 depends on gwtVersion 2.8.0 and this version no longer supports this flag so remove this flag.