Extract DiagnosticReporter implementation from Compiler.

tests/compiler/dart2js/compiler_test.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.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
-import "package:compiler/src/diagnostics/messages.dart";
 import "package:compiler/src/elements/elements.dart";
 import "package:compiler/src/resolution/members.dart";
 import "package:compiler/src/diagnostics/diagnostic_listener.dart";
 import "mock_compiler.dart";
+import "diagnostic_reporter_helper.dart";
 
 
 class CallbackMockCompiler extends MockCompiler {
-  CallbackMockCompiler() : super.internal();
+  CallbackReporter reporter;
+
+  CallbackMockCompiler() : super.internal() {
+    reporter = new CallbackReporter(super.reporter);
+  }
+
+}
+
+class CallbackReporter extends DiagnosticReporterWrapper {
+  final DiagnosticReporter reporter;
+
+  CallbackReporter(this.reporter);
 
   var onError;
   var onWarning;
 
   setOnError(var f) => onError = f;
   setOnWarning(var f) => onWarning = f;
 
   void reportWarning(
       DiagnosticMessage message,
       [List<DiagnosticMessage> infos = const <DiagnosticMessage>[]]) {
@@ skipping 14 matching lines @@
 }
 
 Future testErrorHandling() {
   // Test that compiler.currentElement is set correctly when
   // reporting errors/warnings.
   CallbackMockCompiler compiler = new CallbackMockCompiler();
   return compiler.init().then((_) {
     ResolverVisitor visitor = compiler.resolverVisitor();
     compiler.parseScript('NoSuchPrefix.NoSuchType foo() {}');
     FunctionElement foo = compiler.mainApp.find('foo');
-    compiler.setOnWarning(
+    compiler.reporter.setOnWarning(
         (c, n, m) => Expect.equals(foo, compiler.currentElement));
     foo.computeType(compiler.resolution);
     Expect.equals(1, compiler.warnings.length);
   });
 }
 
 main() {
   asyncTest(() => testErrorHandling());
 }