[parser] only parse async arrow function when necessary

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 2cf04fa568a73fadcdc0cef3c4c8a1f8a105140e..dc08f5c5803923dd9ee29bf92228af10d5f0efe3 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -669,13 +669,13 @@ class ParserBase : public Traits {
     Expect(Token::SEMICOLON, ok);
   }
 
-  bool peek_any_identifier() {
-    Token::Value next = peek();
-    return next == Token::IDENTIFIER || next == Token::ENUM ||
-           next == Token::AWAIT || next == Token::ASYNC ||
-           next == Token::FUTURE_STRICT_RESERVED_WORD || next == Token::LET ||
-           next == Token::STATIC || next == Token::YIELD;
+  bool is_any_identifier(Token::Value token) {
+    return token == Token::IDENTIFIER || token == Token::ENUM ||
+           token == Token::AWAIT || token == Token::ASYNC ||
+           token == Token::FUTURE_STRICT_RESERVED_WORD || token == Token::LET ||
+           token == Token::STATIC || token == Token::YIELD;
   }
+  bool peek_any_identifier() { return is_any_identifier(peek()); }
 
   bool CheckContextualKeyword(Vector<const char> keyword) {
     if (PeekContextualKeyword(keyword)) {
@@ -888,6 +888,10 @@ class ParserBase : public Traits {
     }
   }
 
+  bool IsValidArrowFormalParametersStart(Token::Value token) {
+    return is_any_identifier(token) || token == Token::LPAREN;
+  }
+
   void ValidateArrowFormalParameters(const ExpressionClassifier* classifier,
                                      ExpressionT expr,
                                      bool parenthesized_formals, bool is_async,
@@ -2231,7 +2235,8 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
                                                 classifier->duplicate_finder());
 
   bool is_async = allow_harmony_async_await() && peek() == Token::ASYNC &&
-                  !scanner()->HasAnyLineTerminatorAfterNext();
+                  !scanner()->HasAnyLineTerminatorAfterNext() &&
+                  IsValidArrowFormalParametersStart(PeekAhead());
 
   bool parenthesized_formals = peek() == Token::LPAREN;
   if (!is_async && !parenthesized_formals) {