[parser] Immediately throw if () isn't followed by =>

Change-Id: Iefea33f47ee4492c9c4d4e105e30f7e2d9128bc7
Reviewed-on: https://chromium-review.googlesource.com/c/1319576Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57268}

src/parsing/parser-base.h

@@ -1772,11 +1772,9 @@ ParserBase<Impl>::ParsePrimaryExpression() {
       if (pattern_error) ArrowFormalParametersUnexpectedToken();
       Consume(Token::LPAREN);
       if (Check(Token::RPAREN)) {
-        // ()=>x.  The continuation that looks for the => is in
+        // ()=>x.  The continuation that consumes the => is in
         // ParseAssignmentExpression.
-        classifier()->RecordExpressionError(scanner()->location(),
-                                            MessageTemplate::kUnexpectedToken,
-                                            Token::String(Token::RPAREN));
+        if (peek() != Token::ARROW) ReportUnexpectedToken(Token::RPAREN);
         return factory()->NewEmptyParentheses(beg_pos);
       }
       // Heuristically try to detect immediately called functions before