Filter out core and auxilary libs.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index bae2f85a9610a1af78cd1e8b18c8ff843c62af8a..f997fefb579d076a84a25033dd66f104b9071b48 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -60,10 +60,15 @@ class DartBackend extends Backend {
      * Tells whether we should output given element. Corelib classes like
      * Object should not be in the resulting code.
      */
-    bool shouldOutput(Element element) {
-      return element.kind !== ElementKind.VOID
-          && element.getLibrary() !== compiler.coreLibrary;
-    }
+    final LIBS_TO_IGNORE = [
+      compiler.coreImplLibrary,
+      compiler.jsHelperLibrary,
+      compiler.interceptorsLibrary,
+      compiler.coreLibrary,
+    ];
+    bool shouldOutput(Element element) =>
+      element.kind !== ElementKind.VOID &&
+      LIBS_TO_IGNORE.indexOf(element.getLibrary()) == -1;
 
     try {
       Emitter emitter = new Emitter(compiler);