code: function changeData(state, sourceName, data) {
    const source = state.sources[sourceName];
    if (!source) {
      return state;
    }
    const src_mixin = {};
  
    // update the individual source.
    src_mixin[sourceName] = Object.assign({}, source, {
      data,
    });
  
    // kick back the new state.
    return Object.assign({}, state, {
      sources: Object.assign({}, state.sources, src_mixin),
    }, incrementVersion(state.metadata, dataVersionKey(sourceName)));
  }
docs: Creates a new state with the data for a
  source changed to the contents of data.
  
  @param {Object} state The redux state.
  @param {string} sourceName The name of the source to be changed.
  @param {Object[]} data The list of features to be added to the source.
  
  @returns {Object} The new redux state.