Use DiagnosticMessage in MockCompiler.

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/elements/elements.dart";
 import "package:compiler/src/resolution/members.dart";
 import "package:compiler/src/diagnostics/diagnostic_listener.dart";
@@ skipping 44 matching lines @@
   // 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.reporter.setOnWarning(
         (c, n, m) => Expect.equals(foo, compiler.currentElement));
     foo.computeType(compiler.resolution);
-    Expect.equals(1, compiler.warnings.length);
+    Expect.equals(1, compiler.diagnosticCollector.warnings.length);
   });
 }
 
 main() {
   asyncTest(() => testErrorHandling());
 }