Avoid modifying the real context chain for debug evaluation.

test/mjsunit/debug-evaluate-locals-capturing.js

+// Copyright 2014 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
+
+Debug = debug.Debug
+var exception = null;
+var break_count = 0;
+
+var f = null;
+var i = null;
+function listener(event, exec_state, event_data, data) {
+  try {
+    if (event == Debug.DebugEvent.Break) {
+      var frameMirror = exec_state.frame(0);
+
+      f = frameMirror.evaluate('f = function() { i = 5; }, f(), f').value();
+      print(f);
+    }
+  } catch(e) {
+    exception = e;
+    print(e, e.stack);
+  }
+};
+
+Debug.setListener(listener);
+
+(function (){
+
+  var i = 0;
+
+  try {
+    throw new Error();
+  } catch (e) {
+    assertEquals(0, i);
+    debugger;
+    assertEquals(5, i);
+  }
+}());
+
+assertNull(exception);
+
+assertNull(i);
+f();
+assertNull(i);
+
+Debug.setListener(null);