dart2js: Use frequency of occurence to sort metadata indices.

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

Index: pkg/compiler/lib/src/js_backend/namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/namer.dart b/pkg/compiler/lib/src/js_backend/namer.dart
index 4e15e5d3e00dbf2521b3a692de151a9562a7e4f9..87db683b408825bd15e86c68cb2b83bbc2864055 100644
--- a/pkg/compiler/lib/src/js_backend/namer.dart
+++ b/pkg/compiler/lib/src/js_backend/namer.dart
@@ -1531,7 +1531,17 @@ class ConstantNamingVisitor implements ConstantValueVisitor {
 
   @override
   void visitDummy(DummyConstantValue constant, [_]) {
-    add('dummy_receiver');
+    switch (constant.kind) {
+      case DummyConstantKinds.dummyReceiver:
+        add('dummy_receiver');
+        break;
+      case DummyConstantKinds.typeVariableReference:
+        add('type_variable_reference');
+        break;
+      default:
+        compiler.internalError(compiler.currentElement,
+                               "Unexpected DummyConstantValue");
+    }
   }
 
   @override
@@ -1634,8 +1644,15 @@ class ConstantCanonicalHasher implements ConstantValueVisitor<int, Null> {
 
   @override
   visitDummy(DummyConstantValue constant, [_]) {
-    compiler.internalError(NO_LOCATION_SPANNABLE,
-        'DummyReceiverConstant should never be named and never be subconstant');
+    switch (constant.kind) {
+      case DummyConstantKinds.typeVariableReference:
+        return constant.payload.hashCode;
+      default:
+        compiler.internalError(NO_LOCATION_SPANNABLE,
+                               'DummyConstantValue should never be named and '
+                               'never be subconstant');
+        return null;
+    }
   }
 
   @override