Fix analyzer warning by updating _MockSdkSource.

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Review URL: https://codereview.chromium.org//2278593004 .

Author: stereotype441

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;