Add support for compiling Dart via the FE in dart2js.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2016, 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 mock_compiler;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler_new.dart' as api;
@@ skipping 34 matching lines @@
 import 'diagnostic_helper.dart';
 
 export 'diagnostic_helper.dart';
 
 final Uri PATCH_CORE = new Uri(scheme: 'patch', path: 'core');
 
 typedef String LibrarySourceProvider(Uri uri);
 
 class MockCompiler extends Compiler {
   api.CompilerDiagnostics diagnosticHandler;
+  final api.CompilerInput provider = null;
 
   /// Expected number of warnings. If `null`, the number of warnings is
   /// not checked.
   final int expectedWarnings;
 
   /// Expected number of errors. If `null`, the number of errors is not checked.
   final int expectedErrors;
   final Map<String, SourceFile> sourceFiles;
   Node parsedTree;
   final LibrarySourceProvider librariesOverride;
@@ skipping 329 matching lines @@
       trustTypeAnnotations: trustTypeAnnotations,
       enableTypeAssertions: enableTypeAssertions,
       enableUserAssertions: enableUserAssertions,
       expectedErrors: expectedErrors,
       expectedWarnings: expectedWarnings,
       outputProvider: outputProvider);
   compiler.registerSource(uri, code);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler;
 }