Fix some warnings.

lib/compiler/implementation/dart_backend/backend.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.
 
 class ElementAst {
   final Node ast;
   final TreeElements treeElements;
 
   ElementAst(this.ast, this.treeElements);
 
@@ skipping 114 matching lines @@
    *    or type variable bounds typedefs.
    * These restrictions can be less strict.
    */
   bool isSafeToRemoveTypeDeclarations(
       Map<ClassElement, Set<Element>> classMembers) {
     Set<DartType> processedTypes = new Set<DartType>();
     List<DartType> workQueue = new List<DartType>();
     workQueue.addAll(
         classMembers.getKeys().map((classElement) => classElement.type));
     workQueue.addAll(compiler.resolverWorld.isChecks);
-    DartType typeErrorType =
-        compiler.coreLibrary.find(new SourceString('TypeError')).type;
+    Element typeErrorElement =
+        compiler.coreLibrary.find(new SourceString('TypeError'));
+    DartType typeErrorType = typeErrorElement.computeType(compiler);
     if (workQueue.indexOf(typeErrorType) != -1) {
       return false;
     }
 
     void processTypeArguments(Element classElement, NodeList typeArguments) {
       if (typeArguments == null) return;
       for (Node typeArgument in typeArguments.nodes) {
         if (typeArgument is TypeVariable) {
           typeArgument = typeArgument.bound;
         }
@@ skipping 386 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);