Move component class instantiation into ReactCompositeComponent

We need to move instantiation into the mount phase for context purposes.

To do this I moved the shallow rendering stuff into ReactTestUtils and
reused more of the existing code for it by instantiating a noop child.

Everywhere we refer to the "type" we should pass it to ReactNativeComponent
to resolve any string value into the underlying composite.

src/core/ReactCompositeComponent.js

@@ -108,7 +108,6 @@ var ReactCompositeComponentMixin = {
    */
   construct: function(element) {
     this._currentElement = element;
-
     this._rootNodeID = null;
     this._instance = null;