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

test/mjsunit/regress/regress-325676.js

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-// Flags: --expose-debug-as debug
+// Flags: --expose-debug-as debug --debug-eval-readonly-locals
 
 // If a function parameter is forced to be context allocated,
 // debug evaluate need to resolve it to a context slot instead of
 // parameter slot on the stack.
 
 var Debug = debug.Debug;
 
 var expected;
 var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     assertEquals(expected, exec_state.frame(0).evaluate('arg').value());
-    exec_state.frame(0).evaluate('arg = "evaluated";');
+    exec_state.frame(0).evaluate('arg = "evaluated";');  // no effect
   } catch (e) {
     exception = e;
   }
 }
 
 Debug.setListener(listener);
 
 function f(arg) {
   expected = arg;
   debugger;
-  assertEquals("evaluated", arg);
+  assertEquals(expected, arg);
 
   arg = "value";
   expected = arg;
   debugger;
-  assertEquals("evaluated", arg);
+  assertEquals(expected, arg);
 
   // Forces arg to be context allocated even though a parameter.
   function g() { arg; }
 }
 
 f();
 f(1);
 f(1, 2);
 
 assertNull(exception);