Add DartSdk.getLinkedBundle() and use it in PubSummaryManager.

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';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer/src/summary/idl.dart' show PackageBundle;
 
 class MockSdk implements DartSdk {
   static const _MockSdkLibrary LIB_CORE = const _MockSdkLibrary(
       'dart:core',
       '/lib/core/core.dart',
       '''
 library dart.core;
 
 import 'dart:async';
 import 'dart:_internal';
@@ skipping 286 matching lines @@
           return file.createSource(dartUri);
         } catch (exception) {
           return null;
         }
       }
     }
     return null;
   }
 
   @override
+  PackageBundle getLinkedBundle() => null;
+
+  @override
   SdkLibrary getSdkLibrary(String dartUri) {
     // getSdkLibrary() is only used to determine whether a library is internal
     // to the SDK.  The mock SDK doesn't have any internals, so it's safe to
     // return null.
     return null;
   }
 
   @override
   Source mapDartUri(String dartUri) {
     const Map<String, String> uriToPath = const {
@@ skipping 42 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();
 }