auto-disable preDexLibraries on CI systems

preDexing helps repeat builds run faster, but slows down builds that do
not have any caching.  CI builds start from scratch each time.  Turns
out that GitLab CI, Travis CI, Circle CI, and probably many others all
define the "CI" environment variable, so its easy to detect when running
in a CI environment.  This makes things a lot cleaner.

* https://docs.gitlab.com/ce/ci/variables/README.html
* https://docs.travis-ci.com/user/environment-variables/
* https://circleci.com/docs/2.0/env-vars/
* https://github.com/codepath/android_guides/wiki/Setting-up-Travis-CI
* https://stackoverflow.com/questions/23137764/building-a-debug-apk

.gitlab-ci.yml

@@ -22,13 +22,13 @@ test:
     - ./tools/remove-unused-and-blank-translations.py
     - echo "These are unused or blank translations that should be removed:"
     - git --no-pager diff --exit-code || export EXITVALUE=1
-    - ./gradlew assemble -PdisablePreDex
+    - ./gradlew assemble
     # always report on lint errors to the build log
     - sed -i -e 's,textReport .*,textReport true,' app/build.gradle
-    - ./gradlew lint -PdisablePreDex
-    - ./gradlew pmd -PdisablePreDex
-    - ./gradlew checkstyle -PdisablePreDex
-    - ./gradlew test -PdisablePreDex || {
+    - ./gradlew lint
+    - ./gradlew pmd
+    - ./gradlew checkstyle
+    - ./gradlew test || {
           for log in app/build/reports/*ests/*/*ml; do
               echo "read $log here:";
               (cat "$log" | curl --silent -F 'clbin=<-' https://clbin.com) || true;
@@ -42,12 +42,12 @@ connected10:
   variables:
     AVD_SDK: "10"
   script:
-    - ./gradlew assembleDebug -PdisablePreDex
+    - ./gradlew assembleDebug
     - emulator64-arm -avd fcl-test-$AVD_SDK -no-skin -no-audio -no-window &
     - ./tools/wait-for-emulator
     - adb shell input keyevent 82 &
     - export EXITVALUE=0
-    - ./gradlew connectedCheck -PdisablePreDex || {
+    - ./gradlew connectedCheck || {
         adb -e logcat -d '*:E';
         echo "get the full logcat here:";
         (adb -e logcat -d | curl --silent -F 'clbin=<-' https://clbin.com) || true;
@@ -64,14 +64,14 @@ connected24:
   variables:
     AVD_SDK: "24"
   script:
-    - ./gradlew assembleDebug -PdisablePreDex
+    - ./gradlew assembleDebug
     - android list avd
     - emulator64-arm -avd fcl-test-$AVD_SDK -no-audio -no-window &
     - ./tools/wait-for-emulator
     - adb shell input keyevent 82 &
     - adb devices
     - export EXITVALUE=0
-    - ./gradlew connectedCheck -PdisablePreDex || {
+    - ./gradlew connectedCheck || {
         adb -e logcat -d '*:E';
         echo "get the full logcat here:";
         (adb -e logcat -d | curl --silent -F 'clbin=<-' https://clbin.com) || true;

app/build.gradle

@@ -182,6 +182,9 @@ if (!hasProperty('sourceDeps')) {
     }
 }
 
+def isCi = "true".equals(System.getenv("CI"))
+def preDexEnabled = "true".equals(System.getProperty("pre-dex", "true"))
+
 android {
     compileSdkVersion 24
     buildToolsVersion '25.0.2'
@@ -211,6 +214,13 @@ android {
         cruncherEnabled = false
     }
 
+    dexOptions {
+        // Improve build server performance by allowing disabling of pre-dexing
+        // see http://tools.android.com/tech-docs/new-build-system/tips#TOC-Improving-Build-Server-performance
+        // Skip pre-dexing when running on CI or when disabled via -Dpre-dex=false.
+        preDexLibraries = preDexEnabled && !isCi
+    }
+
     defaultConfig {
         versionCode 1000011
         versionName getVersionName()

build.gradle

@@ -8,19 +8,3 @@ buildscript {
         classpath files('libs/gradle-witness.jar')
     }
 }
-
-/**
- * Improve build server performance by allowing disabling of pre-dexing
- * (see http://tools.android.com/tech-docs/new-build-system/tips#TOC-Improving-Build-Server-performance.)
- */
-project.ext.preDexLibs = !project.hasProperty('disablePreDex')
-
-subprojects {
-  project.plugins.whenPluginAdded { plugin ->
-    if ("com.android.build.gradle.AppPlugin".equals(plugin.class.name)) {
-      project.android.dexOptions.preDexLibraries = rootProject.ext.preDexLibs
-    } else if ("com.android.build.gradle.LibraryPlugin".equals(plugin.class.name)) {
-      project.android.dexOptions.preDexLibraries = rootProject.ext.preDexLibs
-    }
-  }
-}