Classes: Add support for stepping through default constructors

test/mjsunit/harmony/debug-step-into-class-extends.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 --harmony-classes
+
+'use strict';
+
+var Debug = debug.Debug
+
+var done = false;
+var stepCount = 0;
+
+function listener(event, execState, eventData, data) {
+  if (event == Debug.DebugEvent.Break) {
+    if (!done) {
+      execState.prepareStep(Debug.StepAction.StepInto);
+      var s = execState.frame().sourceLineText();
+      assertTrue(s.indexOf('// ' + stepCount + '.') !== -1);
+      stepCount++;
+    }
+  }
+};
+
+Debug.setListener(listener);
+
+function GetBase() {
+  var x = 1;   // 1.
+  var y = 2;   // 2.
+  done = true; // 3.
+  return null;
+}
+
+function f() {
+  class Derived extends GetBase() {} // 0.
+}
+
+var bp = Debug.setBreakPoint(f, 0);
+f();
+assertEquals(4, stepCount);
+
+Debug.setListener(null);