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

pkg/analyzer/lib/src/summary/summary_sdk.dart

Index: pkg/analyzer/lib/src/summary/summary_sdk.dart
diff --git a/pkg/analyzer/lib/src/summary/summary_sdk.dart b/pkg/analyzer/lib/src/summary/summary_sdk.dart
index 1ac305ec0004f47deab935f154f9f092196be7c3..1ed904c4077eadb799510a968d6de6c713ff4985 100644
--- a/pkg/analyzer/lib/src/summary/summary_sdk.dart
+++ b/pkg/analyzer/lib/src/summary/summary_sdk.dart
@@ -6,6 +6,7 @@ library analyzer.src.summary.summary_sdk;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:analyzer/file_system/file_system.dart' show ResourceProvider;
 import 'package:analyzer/src/context/cache.dart' show CacheEntry;
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/dart/element/type.dart';
@@ -20,7 +21,6 @@ import 'package:analyzer/src/summary/package_bundle_reader.dart';
 import 'package:analyzer/src/summary/resynthesize.dart';
 import 'package:analyzer/src/task/dart.dart';
 import 'package:analyzer/task/model.dart' show ResultDescriptor, TargetedResult;
-import 'package:analyzer/file_system/file_system.dart' show ResourceProvider;
 
 class SdkSummaryResultProvider extends ResynthesizerResultProvider {
   final SummaryTypeProvider typeProvider = new SummaryTypeProvider();
@@ -169,6 +169,9 @@ class SummaryBasedDartSdk implements DartSdk {
   }
 
   @override
+  PackageBundle getLinkedBundle() => _bundle;
+
+  @override
   SdkLibrary getSdkLibrary(String uri) {
     // This is not quite correct, but currently it's used only in
     // to report errors on importing or exporting of internal libraries.