Update to support analyzer version 0.28.0.

lib/src/dart_sdk.dart

Index: lib/src/dart_sdk.dart
diff --git a/lib/src/dart_sdk.dart b/lib/src/dart_sdk.dart
index 7f2cb50037953bcb87994bf338712c5fe5257bcf..801de33bb0b179463df320a7fda6dc2a78f888d9 100644
--- a/lib/src/dart_sdk.dart
+++ b/lib/src/dart_sdk.dart
@@ -15,6 +15,7 @@ import 'package:analyzer/src/generated/java_io.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/sdk_io.dart' show DirectoryBasedDartSdk;
 import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer/src/summary/idl.dart';
 import 'package:cli_util/cli_util.dart' as cli_util;
 
 /// Attempts to provide the current Dart SDK directory.
@@ -173,6 +174,9 @@ class MockDartSdk implements DartSdk {
   Source fromFileUri(Uri uri) {
     throw new UnsupportedError('MockDartSdk.fromFileUri');
   }
+
+  @override
+  PackageBundle getLinkedBundle() => null;
 }
 
 class _MockSdkSource implements UriAnnotatedSource {