[debugger] add test case for stepping into tail calls.

test/mjsunit/es6/debug-stepin-tailcalls.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 --harmony-tailcalls
+
+"use strict";
+
+var Debug = debug.Debug
+var exception = null;
+var breaks = 0;
+
+function f(x) {
+  if (x > 0) {            // B3 B5 B7 B9
+    return f(x - 1);      // B4 B6 B8
+  }
+}                         // B10
+
+function g(x) {
+  return f(x);            // B2
+}
+
+function h(x) {
+  debugger;               // B0
+  g(x);                   // B1
+}                         // B11
+
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    print(event_data.sourceLineText());
+    assertTrue(event_data.sourceLineText().indexOf(`B${breaks++}`) > 0);
+    exec_state.prepareStep(Debug.StepAction.StepIn);
+  } catch (e) {
+    exception = e;
+  };
+};
+
+Debug.setListener(listener);
+
+h(3);
+
+Debug.setListener(null);  // B12
+assertNull(exception);
+assertEquals(13, breaks);