anchor.diff:      // Analyzer expects that sources which are referenced but do not exist yet
       // still exist, so just make an empty source.
       if (source == null) {
  -      source = new AssetBasedSource(assetId, _resolver);
  +      source = new AssetBasedSource(assetId);
         _resolver.sources[assetId] = source;
       }
       return source;
  
positive.code:       visiting.add(buildStep.readAsString(assetId).then((contents) {
          var source = sources[assetId];
          if (source == null) {
            source = new AssetBasedSource(assetId, this);
            sources[assetId] = source;
          }
          source.updateDependencies(contents);
  
negative.code:   /// The file contents.
    String _contents;
  
    AssetBasedSource(this.assetId);
  
    /// Update the dependencies of this source. This parses [contents] but avoids
    /// any analyzer resolution.
  