dart2js: Use frequency of occurence to sort metadata indices.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index ce403ad44d6edb0b5f439f11c65f0e5d20c06de4..475d9fd618566aeb5256626beb6a64aabf63ea1d 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -212,6 +212,12 @@ class FunctionInlineCache {
   }
 }
 
+class SyntheticConstantKind {
+  static const String DUMMY_INTERCEPTOR = "DUMMY_INTERCEPTOR";
+  static const String EMPTY_VALUE = "EMPTY_VALUE";
+  static const String TYPEVARIABLE_REFERENCE = "TYPEVARIABLE_REFERENCE";
+}
+
 class JavaScriptBackend extends Backend {
   static final Uri DART_JS_HELPER = new Uri(scheme: 'dart', path: '_js_helper');
   static final Uri DART_INTERCEPTORS =
@@ -1420,6 +1426,7 @@ class JavaScriptBackend extends Backend {
         compiler.enqueuer.codegen.registerStaticUse(getCyclicThrowHelper());
       }
     }
+
     generatedCode[element] = functionCompiler.compile(work);
     return const WorldImpact();
   }