code: function updateSource(state, action) {
    const old_source = state.sources[action.sourceName];
    const new_source = {};
    new_source[action.sourceName] = Object.assign({}, old_source, action.sourceDef);
    const new_sources = Object.assign({}, state.sources, new_source);
  
    let metadata;
    if (new_source[action.sourceName].type === 'geojson') {
      metadata = incrementVersion(state.metadata, dataVersionKey(action.sourceName));
    } else {
      metadata = incrementVersion(state.metadata, SOURCE_VERSION_KEY);
    }
    return Object.assign({}, state, {
      sources: Object.assign({}, state.sources, new_sources),
    }, metadata);
  }
docs: Update a source's definition.
  This is a heavy-handed operation that will
  just mixin whatever is in the new object.
  @param {Object} state Current state.
  @param {Object} action Action to handle.
  
  @returns {Object} The new state.