[cctest] Add a bit of test coverage for disallowing `await` in async params

This enforces that we use ParseExpressionCoverGrammar, and not ParseExpression,
for several tricky cases. Also clarify comment on ParserBase::ParseExpression().

Change-Id: I1d1289abdf539c96f4b42f97c79a8adf3c06e728
Reviewed-on: https://chromium-review.googlesource.com/783171
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49647}

src/parsing/parser-base.h

@@ -1082,8 +1082,8 @@ class ParserBase {
 
   // This method wraps the parsing of the expression inside a new expression
   // classifier and calls RewriteNonPattern if parsing is successful.
-  // It should be used whenever we're parsing an expression that will be
-  // used as a non-pattern (i.e., in most cases).
+  // It should be used whenever we're parsing an expression that is known
+  // to not be a pattern or part of a pattern.
   V8_INLINE ExpressionT ParseExpression(bool accept_IN, bool* ok);
 
   // This method does not wrap the parsing of the expression inside a

test/cctest/test-parsing.cc

@@ -8638,6 +8638,11 @@ TEST(AsyncAwaitFormalParameters) {
     "x = class await {}",
     "x = 1 ? class await {} : 0",
     "x = async function await() {}",
+
+    "x = y[await]",
+    "x = `${await}`",
+    "x = y()[await]",
+
     nullptr
   };
   // clang-format on