dart2js: Use frequency of occurence to sort metadata indices.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of js_backend;
 
 typedef jsAst.Expression _ConstantReferenceGenerator(ConstantValue constant);
 
 /**
  * Generates the JavaScript expressions for constants.
@@ skipping 245 matching lines @@
                           [typeName]);
   }
 
   @override
   jsAst.Expression visitInterceptor(InterceptorConstantValue constant, [_]) {
     ClassElement interceptorClass = constant.dispatchedType.element;
     return backend.emitter.interceptorPrototypeAccess(interceptorClass);
   }
 
   @override
-  jsAst.Expression visitDummy(DummyConstantValue constant, [_]) {
-    return new jsAst.LiteralNumber('0');
+  jsAst.Expression visitSynthetic(SyntheticConstantValue constant, [_]) {
+    switch (constant.kind) {
+      case SyntheticConstantKind.DUMMY_INTERCEPTOR:
+      case SyntheticConstantKind.EMPTY_VALUE:
+        return new jsAst.LiteralNumber('0');
+      case SyntheticConstantKind.TYPEVARIABLE_REFERENCE:
+        return constant.payload;
+      default:
+        compiler.internalError(NO_LOCATION_SPANNABLE,
+                               "Unexpected DummyConstantKind ${constant.kind}");
+        return null;
+    }
   }
 
   @override
   jsAst.Expression visitConstructed(ConstructedConstantValue constant, [_]) {
     Element element = constant.type.element;
     if (backend.isForeign(element)
         && element.name == 'JS_CONST') {
       StringConstantValue str = constant.fields.values.single;
       String value = str.primitiveValue.slowToString();
       return new jsAst.LiteralExpression(stripComments(value));
@@ skipping 27 matching lines @@
                             [value, argumentList]);
     }
     return value;
   }
 
   @override
   jsAst.Expression visitDeferred(DeferredConstantValue constant, [_]) {
     return constantReferenceGenerator(constant.referenced);
   }
 }