code: function(source){
      for (var i = 0, sourceInfo; sourceInfo = this.sourceValues_[i]; i++)
        if (sourceInfo.source === source)
          return true;
  
      return false;
    }
docs: Removes source from sources list.
  @param {basis.data.ReadOnlyDataset|object|function()} source
  @return {boolean} Returns true if source already added.