[parser] improve inferred function names for async arrow functions

src/parsing/parser-base.h

Index: src/parsing/parser-base.h
diff --git a/src/parsing/parser-base.h b/src/parsing/parser-base.h
index c7e4daf97c2b555056c1e17d9d7d78d09df19f0f..c05cc8762c1e8daa1757937fd09067fc2feae6ad 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -2327,6 +2327,11 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
         ParseAndClassifyIdentifier(&arrow_formals_classifier, CHECK_OK);
     expression = this->ExpressionFromIdentifier(name, position(),
                                                 scanner()->location().end_pos);
+    if (fni_) {
+      // Remove `async` and the argument name from the inferred name.
+      fni_->RemoveLastName();
+      fni_->RemoveLastName();

[Dan Ehrenberg, 2016/08/12 00:39:21]

I'm surprised that you have to remove two names here, while the following arrow
function code does not seem to have to remove any. How does this work?

[caitp, 2016/08/12 01:12:27]

well, the `async` is removed on line 2896. As for "multiple arguments", looks
like it works already due to the FuncNameInferrer::State added in
ParseAssignmentExpression, so all names pushed within an AssignmentExpression
are omit.

I think it may be possible to rework this to use FuncNameInferrer::State, but
would still need to add extra code to prevent it from rewinding the name stack
if something turns out not to be an async function. I'm not sure if that's any
better than how it is now.

Either way, more tests coming.

+    }
   }
 
   if (peek() == Token::ARROW) {
@@ -2883,9 +2888,13 @@ ParserBase<Traits>::ParseLeftHandSideExpression(
         Scanner::Location spread_pos;
         typename Traits::Type::ExpressionList args;
         if (V8_UNLIKELY(is_async)) {
+          int async_index = fni_ ? fni_->LastNameIndex() : -1;
           ExpressionClassifier async_classifier(this);
           args = ParseArguments(&spread_pos, true, &async_classifier, CHECK_OK);
           if (peek() == Token::ARROW) {
+            if (fni_) {
+              fni_->RemoveNameAtIndex(async_index);
+            }
             ValidateBindingPattern(&async_classifier, CHECK_OK);
             if (!async_classifier.is_valid_async_arrow_formal_parameters()) {
               ReportClassifierError(