Do not reflect uninitialized 'let' and 'const' in scope mirrors.

test/mjsunit/harmony/debug-blockscopes.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 463 matching lines @@
                    debug.ScopeType.Block,
                    debug.ScopeType.Local,
                    debug.ScopeType.Script,
                    debug.ScopeType.Global], exec_state);
   CheckScopeContent({x:3,y:5}, 0, exec_state);
   CheckScopeContent({x:3,y:5}, 1, exec_state);
   CheckScopeContent({}, 2, exec_state);
 };
 for_loop_5();
 EndTest();
+
+
+// Uninitialized variables
+BeginTest("Uninitialized 1");
+
+function uninitialized_1() {
+  {
+    debugger;
+    let x = 1;
+  }
+}
+
+listener_delegate = function(exec_state) {
+  CheckScopeChain([debug.ScopeType.Block,
+                   debug.ScopeType.Local,
+                   debug.ScopeType.Script,
+                   debug.ScopeType.Global], exec_state);
+  CheckScopeContent({}, 0, exec_state);

[aandrey, 2014/12/08 13:12:22]

DevTools will not show empty Block scopes. Should I change this on our side?

[Dmitry Lomov (no reviews), 2014/12/08 14:34:09]

I do not know what is best here. This change makes variables invisible if you
break before the declaration. I think it is fine if we just not show empty block
scopes.

Another option would be to add not-yet-defined variables to block scopes with
'undefined' value. 

+};
+uninitialized_1();
+EndTest();