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

pkg/analysis_server/test/mock_sdk.dart

Index: pkg/analysis_server/test/mock_sdk.dart
diff --git a/pkg/analysis_server/test/mock_sdk.dart b/pkg/analysis_server/test/mock_sdk.dart
index 504d536a1736d6971936425cf0f36effdfa61132..18d5316efdf6cd257e2cf1e84b93e464a2500b9e 100644
--- a/pkg/analysis_server/test/mock_sdk.dart
+++ b/pkg/analysis_server/test/mock_sdk.dart
@@ -10,6 +10,7 @@ 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(
@@ -316,6 +317,9 @@ external void printToConsole(String line);
   }
 
   @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