Support user generated custom native JS classes.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

 // Copyright (c) 2014, 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.
 library dart2js.cps_ir.type_propagation;
 
 import 'optimizers.dart';
 
 import '../closure.dart' show
     ClosureClassElement;
 import '../common.dart';
@@ skipping 1071 matching lines @@
   ///
   /// Returns `true` if the node was replaced.
   bool specializeFieldAccess(InvokeMethod node) {
     if (!node.selector.isGetter && !node.selector.isSetter) return false;
     AbstractValue receiver = getValue(getDartReceiver(node));
     Element target =
         typeSystem.locateSingleElement(receiver.type, node.selector);
     if (target is! FieldElement) return false;
     // TODO(asgerf): Inlining native fields will make some tests pass for the
     // wrong reason, so for testing reasons avoid inlining them.
-    if (target.isNative) return false;
+    if (target.isNative || target.isJsInterop) return false;
     Continuation cont = node.continuation.definition;
     if (node.selector.isGetter) {
       GetField get = new GetField(getDartReceiver(node), target);
       LetPrim let = makeLetPrimInvoke(get, cont);
       replaceSubtree(node, let);
       push(let);
       return true;
     } else {
       if (target.isFinal) return false;
       assert(cont.parameters.single.hasNoUses);
@@ skipping 1908 matching lines @@
   processLetPrim(LetPrim node) {
     node.primitive.type = null;
     values[node.primitive] = null;
   }
 
   processLetMutable(LetMutable node) {
     node.variable.type = null;
     values[node.variable] = null;
   }
 }