Introduce CodeBuffer as StringBuffer replacement in compiler.

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 BailoutException {
   final String reason;
 
   const BailoutException(this.reason);
 }
 
@@ skipping 10 matching lines @@
       unparseValidator = new UnparseValidator(compiler, validateUnparse),
       resolvedClassMembers = new Map<ClassElement, Set<Element>>(),
       super(compiler) {
     tasks.add(unparseValidator);
   }
 
   void enqueueHelpers(Enqueuer world) {
     // TODO(antonm): Implement this method, if needed.
   }
 
-  CodeBlock codegen(WorkItem work) { return new CodeBlock(null, null); }
+  CodeBuffer codegen(WorkItem work) { return new CodeBuffer(); }
 
   void processNativeClasses(Enqueuer world,
                             Collection<LibraryElement> libraries) {
   }
 
   /**
    * Adds given class element with its member element to resolved classes
    * collections.
    */
   void addMemberToClass(Element element, ClassElement classElement) {
@@ skipping 86 matching lines @@
       compiler.assembledCode = '''
 main() {
   final bailout_reason = "${e.reason}";
 }
 ''';
     }
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }