[debugger] debug-evaluate should not not modify local values.

test/mjsunit/debug-evaluate-modify-catch-block-scope.js

 // Copyright 2015 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
+// Flags: --expose-debug-as debug --debug-eval-readonly-locals
 
 Debug = debug.Debug
 
 var exception = null;
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     exec_state.frame(0).evaluate("bar()");
   } catch (e) {
     exception = e;
     print(e + e.stack);
   }
 }
 
 Debug.setListener(listener);
 
 (function() {
   "use strict";
   try {
     throw 1;
   } catch (e) {
     let a = 1;
     function bar() {
-      a = 2;  // this has no effect, when called from debug-eval
-      e = 2;  // this has no effect, when called from debug-eval.
+      a = 2;
+      e = 2;
     }
     debugger;
-    assertEquals(1, a);
-    assertEquals(1, e);
+    assertEquals(2, a);
+    assertEquals(2, e);
   }
 })();
 
 Debug.setListener(null);
 assertNull(exception);