function ReactPureComponent(props, context, updater) {
  // Duplicated from ReactComponent.
  this.props = props;
  this.context = context;
  this.refs = emptyObject;
  // We initialize the default updater but the real one gets injected by the
  // renderer.
  this.updater = updater || ReactNoopUpdateQueue;
}
Sets a subset of the state. This only exists because _pendingState is
internal. This provides a merging strategy that is not available to deep
properties which is confusing. TODO: Expose pendingState or don't use it
during the merge.

@param {ReactClass} publicInstance The instance that should rerender.
@param {object} partialState Next partial state to be merged with state.
@internal