[debugger] fix debug-evaluate wrt shadowed context var.

test/mjsunit/debug-evaluate-locals.js

 // Copyright 2008 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 101 matching lines @@
     {
       checkFrame0(exec_state.frame(0));
       checkFrame1(exec_state.frame(1));
       checkFrame2(exec_state.frame(2));
 
       // Evaluating a and b on frames 0, 1 and 2 produces 1, 2, 3, 4, 5 and 6.
       assertEquals(1, exec_state.frame(0).evaluate('a').value());
       assertEquals(2, exec_state.frame(0).evaluate('b').value());
       assertEquals(5, exec_state.frame(0).evaluate('eval').value());
       assertEquals(3, exec_state.frame(1).evaluate('a').value());
-      assertEquals(4, exec_state.frame(1).evaluate('b').value());
+      // Reference error because g does not reference b.
+      assertThrows(() => exec_state.frame(1).evaluate('b'), ReferenceError);
       assertEquals("function",
                    typeof exec_state.frame(1).evaluate('eval').value());
       assertEquals(5, exec_state.frame(2).evaluate('a').value());
       assertEquals(6, exec_state.frame(2).evaluate('b').value());
       assertEquals("function",
                    typeof exec_state.frame(2).evaluate('eval').value());
       assertEquals("foo",
                    exec_state.frame(0).evaluate('a = "foo"').value());
       assertEquals("bar",
                    exec_state.frame(1).evaluate('a = "bar"').value());
       // Indicate that all was processed.
       listenerComplete = true;
     }
   } catch (e) {
     exception = e;
     print("Caught something. " + e + " " + e.stack);
   };
 };
 
 // Add the debug event listener.
 Debug.setListener(listener);
 
 var f_result = f();
 
 assertEquals('foobar', f_result);
 
 // Make sure that the debug event listener was invoked.
 assertFalse(exception, "exception in listener")
 assertTrue(listenerComplete);