Add support for fields to the new dart backend.

pkg/compiler/lib/src/dart_backend/renamer.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;
 
 Comparator get _compareNodes =>
     compareBy((n) => n.getBeginToken().charOffset);
 
 abstract class Renamable implements Comparable {
@@ skipping 206 matching lines @@
         (Element element, Set<Node> nodes) {
       _renameNodes(nodes, (_) => _renameGlobal(element));
     });
 
     // Rename locals.
     placeholderCollector.functionScopes.forEach(
         (functionElement, functionScope) {
 
       Set<String> memberIdentifiers = new Set<String>();
       Set<LocalPlaceholder> placeholders = functionScope.localPlaceholders;
-      if (functionElement.enclosingClass != null) {
+      if (functionElement != null && functionElement.enclosingClass != null) {
         functionElement.enclosingClass.forEachMember(
             (enclosingClass, member) {
               memberIdentifiers.add(member.name);
             });
       }
       Set<String> usedLocalIdentifiers = new Set<String>();
       for (LocalPlaceholder placeholder in placeholders) {
         String nextId = _generator.generate(placeholder.identifier, (name) {
           return functionScope.parameterIdentifiers.contains(name)
               || _forbiddenIdentifiers.contains(name)
@@ skipping 115 matching lines @@
   MinifyingGenerator();
 
   String generate(String originalName, bool isForbidden(String name)) {
     String result;
     do {
       result = generateMiniId(index++);
     } while (isForbidden(result));
     return result;
   }
 }