[debugger] flood function for stepping before calling it.

test/mjsunit/debug-stepin-builtin-callback-opt.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
+
+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 replaceCallback(a) {
+  return "x";                                   // Break 2.
+}                                               // Break 3.
+
+var re = /x/g;
+// Optimize the inner helper function for string replace.
+for (var i = 0; i < 10000; i++) "x".replace(re, replaceCallback);
+
+Debug.setListener(listener);
+debugger;                                       // Break 0.
+var result = "x".replace(re, replaceCallback);  // Break 1.
+Debug.setListener(null);                        // Break 4.
+
+assertNull(exception);