react-native-web and react-primitives currently access a few internals
for shimming DOM events into native ones.  Changes in react@16 packaging
hide these internals completely.  This change adds a submodule to react-dom,
unstable-native-dependencies that includes the necessary modules to
continue enabling that method of dom-native event injection.

In order to get some sort of smoke testing on
react-dom-unstable-native-dependencies, update ResponderEventPlugin-test
to use the "public" interfaces provided by react-dom and the new
react-dom/unstable-native dependencies

Also adds the missing references in package.json as well as missing
files required for unittests to do imports correctrly

Also exports injectComponentTree() which is required for the unittests
to re-set the shared component state between runs.