[debugger] flood function for stepping before calling it.

test/mjsunit/es6/debug-step-into-regexp-subclass.js

+// Copyright 2015 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-regexp-subclass
+
+Debug = debug.Debug
+
+var exception = null;
+var break_count = 0;
+
+function listener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    print(event_data.sourceLineText());
+    assertTrue(
+        event_data.sourceLineText().indexOf(`Break ${break_count++}.`) > 0);
+    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+  } catch (e) {
+    exception = e;
+  }
+};
+
+function customSplit() {
+  return "x";              // Break 2.
+}                          // Break 3.
+var o = {};
+o[Symbol.split] = customSplit;
+
+Debug.setListener(listener);
+debugger;                  // Break 0.
+var result = "".split(o);  // Break 1.
+Debug.setListener(null);   // Break 4.
+
+assertEquals("x", result);
+assertNull(exception);