[parser] allow ASI when "await" or "yield" follows "let"

Per https://github.com/tc39/test262/pull/956, André believes that ASI
should be permitted in these situations.

BUG=
R=marja@chromium.org, adamk@chromium.org, littledan@chromium.org

Change-Id: I5602d8a507576607750ffa9e873e1bfa53dd3523
Reviewed-on: https://chromium-review.googlesource.com/472568Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#44585}

src/parsing/parser-base.h

@@ -4132,10 +4132,14 @@ bool ParserBase<Impl>::IsNextLetKeyword() {
                       // for those semantics to apply. This ensures that ASI is
                       // not honored when a LineTerminator separates the
                       // tokens.
-    case Token::YIELD:
-    case Token::AWAIT:
     case Token::ASYNC:
       return true;
+    case Token::AWAIT:
+      // In an async function, allow ASI between `let` and `yield`
+      return !is_async_function() || !scanner_->HasAnyLineTerminatorAfterNext();
+    case Token::YIELD:
+      // In an generator, allow ASI between `let` and `yield`
+      return !is_generator() || !scanner_->HasAnyLineTerminatorAfterNext();
     case Token::FUTURE_STRICT_RESERVED_WORD:
       return is_sloppy(language_mode());
     default:

test/message/let-asi-await-nonasync.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var let = 0;
+function nonAsync() {
+  let
+  await 0;
+}

test/message/let-asi-await-nonasync.out

+*%(basename)s:8: SyntaxError: Unexpected number
+  await 0;
+        ^
+SyntaxError: Unexpected number

test/message/let-asi-yield-nongenerator.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var let = 0;
+function nonGenerator() {
+  let
+  yield 0;
+}

test/message/let-asi-yield-nongenerator.out

+*%(basename)s:8: SyntaxError: Unexpected number
+  yield 0;
+        ^
+SyntaxError: Unexpected number

test/mjsunit/es6/generator-let-asi.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var let = 0;
+function* generator() {
+  let
+  yield 0;
+}
+
+let it = generator();
+let {value, done} = it.next();
+assertEquals(0, value);
+assertEquals(false, done);

test/mjsunit/es8/async-let-asi.js

+// Copyright 2017 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+var let = 0;
+async function asyncFn() {
+  let
+  await 0;
+  return 0;
+}
+
+asyncFn().then(value => {
+  assertEquals(0, value);
+}, error => {
+  assertUnreachable();
+}).catch(error => {
+  %AbortJS(String(error));
+});
+
+%RunMicrotasks();