Compile R8 main source set with source/target 11.

Bug: b/229695990
Change-Id: Ie9fbc266aa190bb2fe5042030e9917230a9e6d1e
diff --git a/build.gradle b/build.gradle
index db4592a..812b438 100644
--- a/build.gradle
+++ b/build.gradle
@@ -646,6 +646,12 @@
 }
 
 setJdkCompilationWithCompatibility(
+        sourceSets.main.compileJavaTaskName,
+        'jdk-11',
+        JavaVersion.VERSION_11,
+        false);
+
+setJdkCompilationWithCompatibility(
         sourceSets.examplesJava9.compileJavaTaskName,
         'openjdk-9.0.4',
         JavaVersion.VERSION_1_9,
@@ -1037,7 +1043,7 @@
             input,
             "--output", output,
             "--pg-map-output", output + ".map",
-            "--lib", "third_party/openjdk/openjdk-rt-1.8/rt.jar"
+            "--lib", org.gradle.internal.jvm.Jvm.current().javaHome,
     ] + args + libs.collectMany { ["--lib", it] } + pgConfs.collectMany { ["--pg-conf", it] }
     return baseR8CommandLine(allArgs)
 }