Add access to FutureOr in TypeProviders (and add implementation in mock sdks)

pkg/analysis_server/test/mock_sdk.dart

 // Copyright (c) 2014, 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 testing.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 155 matching lines @@
 
 import 'dart:math';
 
 class Future<T> {
   factory Future(computation()) => null;
   factory Future.delayed(Duration duration, [T computation()]) => null;
   factory Future.value([value]) => null;
   static Future wait(List<Future> futures) => null;
 }
 
+class FutureOr<T> {}
+
 class Stream<T> {}
 abstract class StreamTransformer<S, T> {}
 ''');
 
   static const MockSdkLibrary LIB_COLLECTION = const MockSdkLibrary(
       'dart:collection',
       '/lib/collection/collection.dart',
       '''
 library dart.collection;
 
@@ skipping 247 matching lines @@
   bool get isInternal => shortName.startsWith('dart:_');
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }