[debugger] do not expose temporary variables introduced by the parser.

test/mjsunit/regress/regress-crbug-582048.js

+// Copyright 2016 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: --expose-debug-as debug
+
+var Debug = debug.Debug;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var frame_count = exec_state.frameCount();
+    for (var i = 0; i < frame_count; i++) {
+      var frame = exec_state.frame(i);
+      var scope_count = frame.scopeCount();
+      for (var j = 0; j < scope_count; j++) {
+        var scope = frame.scope(j);
+        assertTrue(scope.scopeObject().property('').isUndefined());
+      }
+    }
+  } catch (e) {
+    print(e, e.stack);
+    exception = e;
+  }
+}
+
+Debug.setListener(listener);
+
+(function(a = 1) { debugger; })();
+
+Debug.setListener(null);