Open packages to google-java-format; JDK 11 build requirement

* The workaround to make google-java-format work described in
  diffplug/spotless#834 requires raising
  the minimum build requirement to JDK 11.
* Runtime compatibility with JDK 8 is ensured by using the
  '--release' compiler flag.

.github/workflows/gradle.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '8', '11', '16' ]
+        java: [ '11', '16' ]
 
     steps:
       - uses: actions/checkout@v2

build.gradle

@@ -107,9 +107,7 @@ javadoc.options.encoding = 'UTF-8'
 jacoco { toolVersion = '0.8.7' }
 
 tasks.withType(JavaCompile) {
-  if (JavaVersion.current() > JavaVersion.VERSION_1_8) {
-    options.release = 8
-  }
+  options.release = 8
   options.compilerArgs += [
     '-Xlint:unchecked',
     '-Xlint:cast',
@@ -161,9 +159,6 @@ javadoc { exclude '**/internal/**' }
 
 dependencies {
   errorprone("com.google.errorprone:error_prone_core:2.7.1")
-  if (JavaVersion.current().isJava8()) {
-    errorproneJavac("com.google.errorprone:javac:9+181-r4173-1")
-  }
 
   antlr 'org.antlr:antlr4'
 

gradle.properties

@@ -0,0 +1,5 @@
+org.gradle.jvmargs=--add-exports jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED \
+  --add-exports jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED