[debugger] Fix debug-evaluate for context-allocated this.

When `this` is context allocated inside a class constructor (strict mode
function), due to an arrow function, debug evaluate was unable to locate
the value. This is quick fix for the issue, which probably deserves a
more general rewrite at some later point in time (with more domain
expertise).

Bug: chromium:760225
Change-Id: I5208d8a202ad69439f60ada480599d0efcdc4ce4
Cq-Include-Trybots: luci.chromium.try:linux-rel,win7-rel
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1687412
Commit-Queue: Yang Guo <yangguo@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Auto-Submit: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62532}

src/debug/debug-scopes.cc

@@ -615,9 +615,16 @@ bool ScopeIterator::VisitContextLocals(const Visitor& visitor,
 bool ScopeIterator::VisitLocals(const Visitor& visitor, Mode mode) const {
   if (mode == Mode::STACK && current_scope_->is_declaration_scope() &&
       current_scope_->AsDeclarationScope()->has_this_declaration()) {
-    Handle<Object> receiver = frame_inspector_ == nullptr
-                                  ? handle(generator_->receiver(), isolate_)
-                                  : frame_inspector_->GetReceiver();
+    // TODO(bmeurer): We should refactor the general variable lookup
+    // around "this", since the current way is rather hacky when the
+    // receiver is context-allocated.
+    auto this_var = current_scope_->AsDeclarationScope()->receiver();
+    Handle<Object> receiver =
+        this_var->location() == VariableLocation::CONTEXT
+            ? handle(context_->get(this_var->index()), isolate_)
+            : frame_inspector_ == nullptr
+                  ? handle(generator_->receiver(), isolate_)
+                  : frame_inspector_->GetReceiver();
     if (receiver->IsOptimizedOut(isolate_) || receiver->IsTheHole(isolate_)) {
       receiver = isolate_->factory()->undefined_value();
     }

test/debugger/regress/regress-crbug-760225.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.
+
+Debug = debug.Debug
+
+let thisValue;
+function listener(event, exec_state, event_data, data) {
+  if (event == Debug.DebugEvent.Break) {
+    thisValue = exec_state.frame(0).evaluate('this').value();
+  }
+};
+
+Debug.setListener(listener);
+
+class Foo {}
+class Bar extends Foo {
+  constructor() {
+    super();
+    var b = () => this;
+    this.c = 'b';  // <-- Inspect 'this' (it will be undefined)
+    debugger;
+  }
+}
+
+new Bar();
+
+Debug.setListener(null);
+
+assertNotEquals(undefined, thisValue);