Extract DiagnosticReporter implementation from Compiler.

tests/compiler/dart2js/semantic_visitor_test.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.semantics_visitor_test;
 
 import 'dart:async';
 import 'dart:mirrors';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
@@ skipping 312 matching lines @@
     Expect.isFalse(element.isErroneous,
         "Element '$method' is not expected to be have parse errors in:\n"
         "${library.compilationUnit.script.text}");
 
     void testAstElement(AstElement astElement) {
       Expect.isNotNull(astElement, "Element '$method' not found in:\n"
                                    "${library.compilationUnit.script.text}");
       ResolvedAst resolvedAst = astElement.resolvedAst;
       SemanticTestVisitor visitor = createVisitor(resolvedAst.elements);
       try {
-        compiler.withCurrentElement(resolvedAst.element, () {
+        compiler.reporter.withCurrentElement(resolvedAst.element, () {
           //print(resolvedAst.node.toDebugString());
           resolvedAst.node.accept(visitor);
         });
       } catch (e, s) {
         Expect.fail("$e:\n$s\nIn test:\n"
                     "${library.compilationUnit.script.text}");
       }
       Expect.listEquals(expectedVisits, visitor.visits,
           "In test:\n"
           "${library.compilationUnit.script.text}\n\n"
@@ skipping 407 matching lines @@
   ERROR_INVALID_INDEX_SET,
   ERROR_INVALID_COMPOUND_INDEX_SET,
   ERROR_INVALID_INDEX_PREFIX,
   ERROR_INVALID_INDEX_POSTFIX,
 
   VISIT_CONSTANT_GET,
   VISIT_CONSTANT_INVOKE,
 
   PREVISIT_DEFERRED_ACCESS,
 }