Support stepping into generator function.

test/mjsunit/es6/debug-stepin-generators.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 exception = null;
+var yields = 0;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    var source = exec_state.frame(0).sourceLineText();
+    print(source);
+    if (/stop stepping/.test(source)) return;
+    if (/yield/.test(source)) yields++;
+    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+  } catch (e) {
+    print(e, e.stack);
+    exception = e;
+  }
+};
+
+Debug.setListener(listener);
+
+function* g() {
+  for (var i = 0; i < 3; ++i) {
+    yield i;
+  }
+}
+
+var i = g();
+debugger;
+for (var num of g()) {}
+i.next();
+
+print(); // stop stepping
+
+// Not stepped into.
+i.next();
+i.next();
+
+assertNull(exception);
+assertEquals(4, yields);