[turbofan] Introduce JSCallForwardVarargs operator.

src/runtime/runtime-compiler.cc

Index: src/runtime/runtime-compiler.cc
diff --git a/src/runtime/runtime-compiler.cc b/src/runtime/runtime-compiler.cc
index f1c76bb2acc5734cc629d26784b5eb58b838d12b..699c4c3a6e8add0ed2d3943d2972b2c5bd975fc5 100644
--- a/src/runtime/runtime-compiler.cc
+++ b/src/runtime/runtime-compiler.cc
@@ -182,6 +182,10 @@ RUNTIME_FUNCTION(Runtime_NotifyDeoptimized) {
     JavaScriptFrameIterator top_it(isolate);
     JavaScriptFrame* top_frame = top_it.frame();
     isolate->set_context(Context::cast(top_frame->context()));
+  } else {
+    // TODO(turbofan): We currently need the native context to materialize
+    // the arguments object, but only to get to its map.
+    isolate->set_context(function->native_context());
   }
 
   // Make sure to materialize objects before causing any allocation.