Extract DiagnosticReporter implementation from Compiler.

tests/compiler/dart2js/backend_dart/dart_backend_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 "package:expect/expect.dart";
 import 'dart:async';
 import "package:async_helper/async_helper.dart";
 import '../mock_compiler.dart';
 import '../mock_libraries.dart';
 import '../output_collector.dart';
@@ skipping 596 matching lines @@
   ElementAst operator[](AstElement element) {
     return new ElementAst(element.resolvedAst.node,
                           element.resolvedAst.elements);
   }
 
   noSuchMethod(Invocation invocation) => throw 'unimplemented method';
 }
 
 PlaceholderCollector collectPlaceholders(compiler, element) {
   DartBackend backend = compiler.backend;
-  return new PlaceholderCollector(compiler,
+  return new PlaceholderCollector(
+      compiler.reporter,
       backend.mirrorRenamer,
       new Set<String>(),
       new DynoMap(compiler),
       compiler.mainFunction)
     ..collect(element);
 }
 
 testLocalFunctionPlaceholder() {
   var src = '''
 main() {
@@ skipping 479 matching lines @@
   testParametersMinified();
   testDeclarationTypePlaceholders();
   testPlatformLibraryMemberNamesAreFixed();
   testConflictsWithCoreLib();
   testUnresolvedNamedConstructor1();
   testUnresolvedNamedConstructor2();
   testUnresolvedNamedConstructor3();
   testClassAndNamedMixinDeclarations();
 }