Make instanceof and Object.getPrototypeOf work for proxies, plus a few other tweaks.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index bf881e99db955d6e88bc4d666e7f0df3f9d194e0..47b964e18edd4dd61c29e5b8a08acefb5b551f3f 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -638,7 +638,7 @@ Object* Object::GetPrototype() {
 
   // The object is either a number, a string, a boolean,
   // a real JS object, or a Harmony proxy.
-  if (heap_object->IsJSObject() || heap_object->IsJSProxy()) {
+  if (heap_object->IsJSReceiver()) {
     return heap_object->map()->prototype();
   }
   Heap* heap = heap_object->GetHeap();
@@ -3345,8 +3345,7 @@ void JSObject::LocalLookup(String* name, LookupResult* result) {
   }
 
   // Check __proto__ before interceptor.
-  if (name->Equals(heap->Proto_symbol()) &&
-      !IsJSContextExtensionObject()) {
+  if (name->Equals(heap->Proto_symbol()) && !IsJSContextExtensionObject()) {
     result->ConstantResult(this);
     return;
   }