Preserve the original object when using replaceState

When we use only replace state we don't need to clone the object.

scripts/fiber/tests-failing.txt

@@ -114,7 +114,6 @@ src/renderers/shared/stack/reconciler/__tests__/ReactCompositeComponent-test.js
 * should warn about `setState` in render
 * should warn about `setState` in getChildContext
 * should update refs if shouldComponentUpdate gives false
-* should support objects with prototypes as state
 
 src/renderers/shared/stack/reconciler/__tests__/ReactCompositeComponentNestedState-test.js
 * should provide up to date values for props

scripts/fiber/tests-passing.txt

@@ -1284,6 +1284,7 @@ src/renderers/shared/stack/reconciler/__tests__/ReactCompositeComponent-test.js
 * should allow access to findDOMNode in componentWillUnmount
 * context should be passed down from the parent
 * should replace state
+* should support objects with prototypes as state
 * should not warn about unmounting during unmounting
 * should only call componentWillUnmount once
 

src/renderers/shared/fiber/ReactFiberUpdateQueue.js

@@ -87,19 +87,30 @@ exports.callCallbacks = function(queue : UpdateQueue, context : any) : Error | n
   return firstError;
 };
 
+function getStateFromNode(node, instance, state, props) {
+  if (typeof node.partialState === 'function') {
+    const updateFn = node.partialState;
+    return updateFn.call(instance, state, props);
+  } else {
+    return node.partialState;
+  }
+}
+
 exports.mergeUpdateQueue = function(queue : UpdateQueue, instance : any, prevState : any, props : any) : any {
   let node : ?UpdateQueueNode = queue;
+  if (queue.isReplace) {
+    // replaceState is always first in the queue.
+    prevState = getStateFromNode(queue, instance, prevState, props);
+    node = queue.next;
+    if (!node) {
+      // If there is no more work, we replace the raw object instead of cloning.
+      return prevState;
+    }
+  }
   let state = Object.assign({}, prevState);
   while (node) {
-    state = node.isReplace ? null : state;
-    let partialState;
-    if (typeof node.partialState === 'function') {
-      const updateFn = node.partialState;
-      partialState = updateFn.call(instance, state, props);
-    } else {
-      partialState = node.partialState;
-    }
-    state = Object.assign(state || {}, partialState);
+    let partialState = getStateFromNode(node, instance, state, props);
+    Object.assign(state, partialState);
     node = node.next;
   }
   return state;