Fix analyzer warning by updating _MockSdkSource.

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 2e95de8e09719b2fa43cdd08c4e020151ac64f2d..7f2cb50037953bcb87994bf338712c5fe5257bcf 100644
--- a/lib/src/dart_sdk.dart
+++ b/lib/src/dart_sdk.dart
@@ -182,6 +182,8 @@ class _MockSdkSource implements UriAnnotatedSource {
 
   Source get source => this;
 
+  Source get librarySource => null;
+
   _MockSdkSource(this.uri, this._contents);
 
   bool exists() => true;