Partial revert of rest parameter desugaring.

src/runtime/runtime-scopes.cc

Index: src/runtime/runtime-scopes.cc
diff --git a/src/runtime/runtime-scopes.cc b/src/runtime/runtime-scopes.cc
index c416e244faabec36df54efd75e4ef2b0d9f3602c..16917f9da5fa337533c192308ef50ad5f82c4a6c 100644
--- a/src/runtime/runtime-scopes.cc
+++ b/src/runtime/runtime-scopes.cc
@@ -605,6 +605,54 @@ RUNTIME_FUNCTION(Runtime_NewStrictArguments) {
 }
 
 
+static Handle<JSArray> NewRestParam(Isolate* isolate, Object** parameters,
+                                    int num_params, int rest_index,
+                                    LanguageMode language_mode) {
+  parameters -= rest_index;
+  int num_elements = std::max(0, num_params - rest_index);
+  Handle<FixedArray> elements =
+      isolate->factory()->NewUninitializedFixedArray(num_elements);
+  for (int i = 0; i < num_elements; ++i) {
+    elements->set(i, *--parameters);
+  }
+  return isolate->factory()->NewJSArrayWithElements(
+      elements, FAST_ELEMENTS, num_elements, strength(language_mode));
+}
+
+
+RUNTIME_FUNCTION(Runtime_NewRestParam) {
+  HandleScope scope(isolate);
+  DCHECK(args.length() == 4);
+  Object** parameters = reinterpret_cast<Object**>(args[0]);
+  CONVERT_SMI_ARG_CHECKED(num_params, 1);
+  CONVERT_SMI_ARG_CHECKED(rest_index, 2);
+  CONVERT_SMI_ARG_CHECKED(language_mode, 3);
+
+  return *NewRestParam(isolate, parameters, num_params, rest_index,
+                       static_cast<LanguageMode>(language_mode));
+}
+
+
+RUNTIME_FUNCTION(Runtime_NewRestParamSlow) {
+  HandleScope scope(isolate);
+  DCHECK(args.length() == 2);
+  CONVERT_SMI_ARG_CHECKED(rest_index, 0);
+  CONVERT_SMI_ARG_CHECKED(language_mode, 1);
+
+  JavaScriptFrameIterator it(isolate);
+
+  // Find the frame that holds the actual arguments passed to the function.
+  it.AdvanceToArgumentsFrame();
+  JavaScriptFrame* frame = it.frame();
+
+  int argument_count = frame->GetArgumentsLength();
+  Object** parameters = reinterpret_cast<Object**>(frame->GetParameterSlot(-1));
+
+  return *NewRestParam(isolate, parameters, argument_count, rest_index,
+                       static_cast<LanguageMode>(language_mode));
+}
+
+
 RUNTIME_FUNCTION(Runtime_NewClosure) {
   HandleScope scope(isolate);
   DCHECK_EQ(1, args.length());