Extract DiagnosticReporter implementation from Compiler.

pkg/compiler/lib/src/js_emitter/full_emitter/code_emitter_helper.dart

 // Copyright (c) 2013, 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 dart2js.js_emitter.full_emitter;
 
 class CodeEmitterHelper {
   Emitter emitter;
 
   Namer get namer => emitter.namer;
 
   JavaScriptBackend get backend => emitter.backend;
 
   CodeEmitterTask get task => emitter.task;
 
   Compiler get compiler => emitter.compiler;
 
+  DiagnosticReporter get reporter => compiler.reporter;
+
   String get n => emitter.n;
 
   String get _ => emitter._;
 
   String get N => emitter.N;
 }