Support enums in dart2dart.

pkg/compiler/lib/src/dart_backend/placeholder_collector.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 dart_backend;
 
 class LocalPlaceholder {
   final String identifier;
   final Set<Node> nodes;
   LocalPlaceholder(this.identifier) : nodes = new Set<Node>();
@@ skipping 327 matching lines @@
   void makeElementPlaceholder(Node node, Element element) {
     assert(node != null);
     assert(element != null);
     LibraryElement library = element.library;
     if (identical(element, mainFunction)) return;
     if (library.isDartCore) return;
 
     if (library.isPlatformLibrary && !element.isTopLevel) {
       return;
     }
+
+    ClassElement cls = element.enclosingClass;
+    if (cls != null && cls.isEnumClass) {
+      // Enums and enum values cannot be changed, since the semantics of
+      // `toString` is defined by the names of the declarations.
+      return;
+    }
+
     if (element.isGetter || element.isSetter) {
       element = (element as FunctionElement).abstractField;
     }
     elementNodes.putIfAbsent(element, () => new Set<Node>()).add(node);
   }
 
   /// Marks [node] to be renamed per-library if it names an instance member
   /// and has a private name.
   void tryMakePrivateIdentifier(Node node, Element element) {
     if (node is Identifier &&
@@ skipping 336 matching lines @@
 
   visitBlock(Block node) {
     for (Node statement in node.statements.nodes) {
       if (statement is VariableDefinitions) {
         makeVarDeclarationTypePlaceholder(statement);
       }
     }
     node.visitChildren(this);
   }
 }