Check for a passed-in type argument vector in the prolog of generic functions.

runtime/vm/native_entry.h

Index: runtime/vm/native_entry.h
diff --git a/runtime/vm/native_entry.h b/runtime/vm/native_entry.h
index 200703f87199837ed75301a9524bd32056716e65..7a5be6e58012b4491aba7bd634b145da8bb2452a 100644
--- a/runtime/vm/native_entry.h
+++ b/runtime/vm/native_entry.h
@@ -53,7 +53,7 @@ typedef void (*NativeFunction)(NativeArguments* arguments);
 #ifdef DEBUG
 #define SET_NATIVE_RETVAL(args, value)                                         \
   RawObject* retval = value;                                                   \
-  ASSERT(retval->IsDartInstance());                                            \
+  ASSERT(retval->IsDartInstance() || retval->IsTypeArguments());               \

[rmacnak, 2017/06/14 00:14:20]

This makes me nervous. Is it possible to use a runtime function instead of a
native function?

[regis, 2017/06/14 21:02:04]

I first looked at a runtime call, but a native one seemed easier to implement. I
can have a second look.

As a side note, we pass TypeArguments as arguments to factory methods already
and we do not have asserts checking that all arguments are DartInstance.

[rmacnak, 2017/06/15 00:03:27]

Ah, I hadn't thought of factory type arguments. I withdraw my concern here.

   arguments->SetReturnUnsafe(retval);
 #else
 #define SET_NATIVE_RETVAL(arguments, value) arguments->SetReturnUnsafe(value);