[parser] Don't mark const variables as assigned

Since const variables are  immutable, ignore SetMaybeAssigned for them.

Bug: chromium:999450, chromium:1000170, v8:8510
Change-Id: Idc1b71677b3d03bb63cc025017c119710b8f392d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1782170
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63579}

src/ast/variables.h

@@ -73,6 +73,8 @@ class Variable final : public ZoneObject {
     return MaybeAssignedFlagField::decode(bit_field_);
   }
   void SetMaybeAssigned() {
+    if (mode() == VariableMode::kConst) return;
+
     // If this variable is dynamically shadowing another variable, then that
     // variable could also be assigned (in the non-shadowing case).
     if (has_local_if_not_shadowed()) {
@@ -81,7 +83,8 @@ class Variable final : public ZoneObject {
       if (!maybe_assigned()) {
         local_if_not_shadowed()->SetMaybeAssigned();
       }
-      DCHECK(local_if_not_shadowed()->maybe_assigned());
+      DCHECK_IMPLIES(local_if_not_shadowed()->mode() != VariableMode::kConst,
+                     local_if_not_shadowed()->maybe_assigned());
     }
     set_maybe_assigned();
   }

test/mjsunit/regress/regress-crbug-1000170.js

+// Copyright 2019 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: --no-lazy --stress-lazy-source-positions --enable-lazy-source-positions
+
+(function a() {
+  function b() { a(); }
+  function c() { eval(); }
+})();

test/mjsunit/regress/regress-crbug-999450.js

+// Copyright 2019 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: --no-lazy --stress-lazy-source-positions --enable-lazy-source-positions
+
+(function foo() {
+    foo = null;
+    () => foo;
+})