function Component(props, context, updater) {
              this.props = props;
              this.context = context;
              this.refs = emptyObject;
              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.
@param {?function} callback Called after component is updated.
@param {?string} Name of the calling function in the public API.
@internal