Allow stepping into Object.observe handlers.

test/mjsunit/debug-stepin-foreach.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
+// Tests stepping into through Array.prototype.forEach callbacks.
+
+Debug = debug.Debug
+var exception = null;
+var break_count = 0;
+var expected_breaks = -1;
+
+function listener(event, exec_state, event_data, data) {
+  try {
+    if (event == Debug.DebugEvent.Break) {
+      assertTrue(exec_state.frameCount() != 0, "FAIL: Empty stack trace");
+      if (!break_count) {
+        // Count number of expected breakpoints in this source file.
+        var source_text = exec_state.frame(0).func().script().source();
+        expected_breaks = source_text.match(/\/\/\s*Break\s+\d+\./g).length;
+        print("Expected breaks: " + expected_breaks);
+      }
+      var source = exec_state.frame(0).sourceLineText();
+      print("paused at: " + source);
+      assertTrue(source.indexOf("// Break " + break_count + ".") > 0,
+                 "Unexpected pause at: " + source + "\n" +
+                 "Expected: // Break " + break_count + ".");
+      ++break_count;
+      if (break_count !== expected_breaks) {
+        exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+      }
+    }
+  } catch(e) {
+    exception = e;
+    print(e, e.stack);
+  }
+};
+
+Debug.setListener(listener);
+
+debugger; // Break 0.
+[1,2].forEach(callback); // Break 1.
+
+function callback(x) {
+  return x; // Break 2. // Break 4.
+} // Break 3. // Break 5.
+
+assertNull(exception); // Break 6.
+assertEquals(expected_breaks, break_count);
+
+Debug.setListener(null);