Refactor findHostInstance and findNodeHandle (#12575)

* Move findNodeHandle into the renderers and use instantiation

This is just like ReactDOM does it. This also lets us get rid of injection
for findNodeHandle. Instead I move NativeMethodsMixin and ReactNativeComponent
to use instantiation.

* Refactor findHostInstance

The reconciler shouldn't expose the Fiber data structure. We should pass
the component instance to the reconciler, since the reconciler is the
thing that is supposed to be instancemap aware.

* Fix devtools injection

Author: sebmarkbage

packages/react-dom/src/__tests__/findDOMNode-test.js

@@ -65,9 +65,7 @@ describe('findDOMNode', () => {
   it('findDOMNode should reject random objects', () => {
     expect(function() {
       ReactDOM.findDOMNode({foo: 'bar'});
-    }).toThrowError(
-      'Element appears to be neither ReactComponent nor DOMNode. Keys: foo',
-    );
+    }).toThrowError('Argument appears to not be a ReactComponent. Keys: foo');
   });
 
   it('findDOMNode should reject unmounted objects with render func', () => {

packages/react-dom/src/client/ReactDOM.js

@@ -1147,20 +1147,7 @@ const ReactDOM: Object = {
       return (componentOrElement: any);
     }
 
-    const inst = ReactInstanceMap.get(componentOrElement);
-    if (inst) {
-      return DOMRenderer.findHostInstance(inst);
-    }
-
-    if (typeof componentOrElement.render === 'function') {
-      invariant(false, 'Unable to find node on an unmounted component.');
-    } else {
-      invariant(
-        false,
-        'Element appears to be neither ReactComponent nor DOMNode. Keys: %s',
-        Object.keys(componentOrElement),
-      );
-    }
+    return DOMRenderer.findHostInstance(componentOrElement);
   },
 
   hydrate(element: React$Node, container: DOMContainer, callback: ?Function) {