Introduce debug events for Microtask queue.

test/mjsunit/es7/object-observe-debug-event.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 base_id = -1;
+var exception = null;
+var expected = [
+  "enqueue #1",
+  "willHandle #1",
+  "didHandle #1",
+];
+
+function assertLog(msg) {
+  print(msg);
+  assertTrue(expected.length > 0);
+  assertEquals(expected.shift(), msg);
+  if (!expected.length) {
+    Debug.setListener(null);
+  }
+}
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.AsyncTaskEvent) return;
+  try {
+    if (base_id < 0)
+      base_id = event_data.id();
+    var id = event_data.id() - base_id + 1;
+    assertEquals("Object.observe", event_data.name());
+    assertLog(event_data.type() + " #" + id);
+  } catch (e) {
+    print(e + e.stack)
+    exception = e;
+  }
+}
+
+Debug.setListener(listener);
+
+var obj = {};
+Object.observe(obj, function(changes) {
+  print(change.type + " " + change.name + " " + change.oldValue);
+});
+
+obj.foo = 1;
+obj.zoo = 2;
+obj.foo = 3;
+
+assertNull(exception);