Build Java8-compatible release (#1586)

* Build Java8-compatible release

Use the new options.release Gradle property to make sure builds are
compatible with Java 8, which is the runtime on Appengine.

This new property replaces sourceCompatibility, targetCompatibility, and
bootclasspath (wasn't previously set, which is the reason why we
couldn't detect Java9 api usage when building).

build.gradle

@@ -331,6 +331,13 @@ subprojects {
 
   apply from: "${rootDir.path}/java_common.gradle"
 
+  if (project.name != 'docs') {
+    compileJava {
+      // TODO: Remove this once we migrate off AppEngine.
+      options.release = 8
+    }
+  }
+
   if (project.name == 'third_party') return
 
   project.tasks.test.dependsOn runPresubmits

docs/build.gradle

@@ -37,9 +37,6 @@ test {
   useJUnitPlatform()
 }
 
-sourceCompatibility = '11'
-targetCompatibility = '11'
-
 task flowDocsTool(type: JavaExec) {
   systemProperty 'test.projectRoot', rootProject.projectRootDir
   jvmArgs = ['--add-exports', 'jdk.javadoc/jdk.javadoc.internal.tool=ALL-UNNAMED']

java_common.gradle

@@ -128,13 +128,6 @@ tasks.withType(JavaCompile).configureEach {
     }
 }
 
-// TODO: Change source version to 11 after target version is changed to 11 and
-// once we figure out what's wrong with Dagger compilation with source version
-// >8.
-sourceCompatibility = '8'
-// TODO: Change target version to 11. Source version can stay at 8.
-targetCompatibility = '8'
-
 compileJava { options.encoding = "UTF-8" }
 compileTestJava { options.encoding = "UTF-8" }