Issue #1918 - m-c 1353690: Properly exclude |await| from consideration as a |let| variable name in async functions, so that a newline between the two separates them into two distinct expressions.

js/src/frontend/Parser.cpp

@@ -7348,6 +7348,11 @@ Parser<ParseHandler>::nextTokenContinuesLetDeclaration(TokenKind next, YieldHand
     if (next == TOK_YIELD)
         return yieldHandling == YieldIsName;
 
+    // Somewhat similar logic applies for "await", except that it's not tracked
+    // with an AwaitHandling argument.
+    if (next == TOK_AWAIT)
+        return !awaitIsKeyword();
+
     // Otherwise a let declaration must have a name.
     if (TokenKindIsPossibleIdentifier(next)) {
         // A "let" edge case deserves special comment.  Consider this:

js/src/tests/test262/local/let-newline-await-in-async-function.js

@@ -0,0 +1,18 @@
+// Copyright (C) 2017 Mozilla Corporation. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+author: Jeff Walden <jwalden+code@mit.edu>
+esid: sec-let-and-const-declarations
+description: >
+  |await| is excluded from LexicalDeclaration by grammar parameter, in
+  AsyncFunction.  Therefore |let| followed by |await| inside AsyncFunction is
+  an ASI opportunity, and this code must parse without error.
+---*/
+
+async function f() {
+    let
+    await 0;
+}
+
+reportCompare(true, f instanceof Function);

js/src/tests/test262/local/let-newline-await-in-normal-function.js

@@ -0,0 +1,20 @@
+// |reftest| error:SyntaxError
+// Copyright (C) 2017 Mozilla Corporation. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+author: Jeff Walden <jwalden+code@mit.edu>
+esid: sec-let-and-const-declarations
+description: >
+  Outside AsyncFunction, |await| is a perfectly cromulent LexicalDeclaration
+  variable name.  Therefore ASI doesn't apply, and so the |0| where a |=| was
+  expected is a syntax error.
+negative:
+  phase: early
+  type: SyntaxError
+---*/
+
+function f() {
+    let
+    await 0;
+}