[debugger] allow debug-evaluate to change stack and context values.

test/mjsunit/debug-evaluate-nested-let.js

+// Copyright 2016 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 --allow-natives-syntax
+
+Debug = debug.Debug
+
+var exception = null;
+
+function f() {
+  let a = 0;
+  function g() {
+    let a = 1;
+    {
+      let a = 2;
+      debugger;  // Breakpoint.
+      if (a !== 3) {
+        // We cannot change stack locals in optimized frames.
+        assertEquals(2, a);
+        assertOptimized(g);
+      }
+    }
+    assertEquals(1, a);
+  }
+  g.call(1);
+  if (a !== 4) {
+    // We cannot change stack locals in optimized frames.
+    assertEquals(0, a);
+    assertOptimized(f);
+  }
+}
+
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    exec_state.frame(0).evaluate("a = 3");
+    exec_state.frame(1).evaluate("a = 4");
+    assertThrows(() => exec_state.frame(0).evaluate("this = 2"));
+  } catch (e) {
+    exception = e;
+    print("Caught something. " + e + " " + e.stack);
+  };
+};
+
+Debug.setListener(listener);
+
+f();
+
+Debug.setListener(null);
+assertNull(exception);