dart2js: Use frequency of occurence to sort metadata indices.

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

Index: pkg/compiler/lib/src/js_backend/constant_emitter.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_emitter.dart b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
index f01f0d079e3ad55a3aae269686186f767e5a95dc..b7f8d867cbfd018947b3a749811b7ae614411273 100644
--- a/pkg/compiler/lib/src/js_backend/constant_emitter.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
@@ -264,7 +264,17 @@ class ConstantEmitter
 
   @override
   jsAst.Expression visitDummy(DummyConstantValue constant, [_]) {
-    return new jsAst.LiteralNumber('0');
+    switch (constant.kind) {
+      case DummyConstantKinds.dummyReceiver:
+      case DummyConstantKinds.emptyValue:
+        return new jsAst.LiteralNumber('0');
+      case DummyConstantKinds.typeVariableReference:
+        return constant.payload;
+      default:
+        compiler.internalError(NO_LOCATION_SPANNABLE,
+                               "Unexpected DummyConstantKind ${constant.kind}");
+        return null;
+    }
   }
 
   @override