[esnext] prototype runtime implementation for async functions

src/frames.cc

Index: src/frames.cc
diff --git a/src/frames.cc b/src/frames.cc
index 2a1a26461d3b032b0efd5afa995cc363e20762e5..b9e39f227aeb54fedc5eb784a298b207c71a6855 100644
--- a/src/frames.cc
+++ b/src/frames.cc
@@ -865,6 +865,14 @@ void JavaScriptFrame::Summarize(List<FrameSummary>* functions,
                                 FrameSummary::Mode mode) const {
   DCHECK(functions->length() == 0);
   Code* code = LookupCode();
+
+  // TODO(caitp): these builtins cause crashes here --- that's not good!
+  // Should probably be a better way to keep functions from being included in
+  // stack trace.
+  if (code == *function()->GetIsolate()->builtins()->AsyncFunctionNext() ||
+      code == *function()->GetIsolate()->builtins()->AsyncFunctionThrow())
+    return;
+
   int offset = static_cast<int>(pc() - code->instruction_start());
   AbstractCode* abstract_code = AbstractCode::cast(code);
   FrameSummary summary(receiver(), function(), abstract_code, offset,