Fix children could not be Iterable.
Fix children could not be Iterable.

If the children wasn't List, it was assumed they need to be packed in one. However, this created issues when custom Iterable of children were used instead of List. This is fixed now.

greglittlefield-wf/react-dart,trentgrover-wf/react-dart,evanweible-wf/react-dart,cleandart/react-dart,greglittlefield-wf/react-dart,csarsam/react-dart-native,trentgrover-wf/react-dart,astashov/react-dart,evanweible-wf/react-dart,cleandart/react-dart,csarsam/react-dart-native,trentgrover-wf/react-dart,evanweible-wf/react-dart,trentgrover-wf/react-dart,cleandart/react-dart,cleandart/react-dart,danschultz/react-dart,astashov/react-dart,greglittlefield-wf/react-dart,danschultz/react-dart,astashov/react-dart,astashov/react-dart,csarsam/react-dart-native,danschultz/react-dart,danschultz/react-dart,evanweible-wf/react-dart