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

test/mjsunit/debug-evaluate-const.js

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-// Flags: --expose-debug-as debug
+// Flags: --expose-debug-as debug --debug-eval-readonly-locals
 
 Debug = debug.Debug
 
 listenerComplete = false;
 exception = false;
 
 // var0:   init after break point, changed by debug eval.
 // const0: init before break point, changed by debug eval.
 // const1: init after break point, materialized but untouched by debug eval.
 // const2: init after break point, materialized and changed by debug eval.
 // const3: context allocated const, init before break point, changed by eval.
 function f() {
   var var1 = 21;
   const const3 = 3;
 
   function g() {
     const const0 = 0;
     assertEquals(undefined, const1);
     assertEquals(undefined, const2);
     assertEquals(3, const3);
     assertEquals(21, var1);
 
     debugger;  // Break point.
 
-    assertEquals(30, var0);
-    // TODO(yangguo): debug evaluate should not be able to alter
-    //                stack-allocated const values
-    // assertEquals(0, const0);
+    assertEquals(undefined, var0);
+    assertEquals(0, const0);
     assertEquals(undefined, const1);
     assertEquals(undefined, const2);
     var var0 = 20;
     const const1 = 1;
     const const2 = 2;
     assertEquals(20, var0);
     assertEquals(1, const1);
     assertEquals(2, const2);
   }
 
   g();
 
-  assertEquals(31, var1);
+  assertEquals(21, var1);
   assertEquals(3, const3);
 }
 
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     var frame = exec_state.frame(0);
     var evaluate = function(something) {
       return frame.evaluate(something).value()
@@ skipping 31 matching lines @@
 try { f(); } catch (e) { }
 
 Debug.setListener(listener);
 
 f();
 
 Debug.setListener(null);
 
 assertFalse(exception, "exception in listener")
 assertTrue(listenerComplete);