Do not break in native code (including non-builtin debugger code).

test/mjsunit/debug-break-native.js

Index: test/mjsunit/debug-break-native.js
diff --git a/test/mjsunit/debug-break-native.js b/test/mjsunit/debug-break-native.js
new file mode 100644
index 0000000000000000000000000000000000000000..11d7274929c5bbc989bb7890e0c07c0d91d0492b
--- /dev/null
+++ b/test/mjsunit/debug-break-native.js
@@ -0,0 +1,42 @@
+// 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
+
+Debug = debug.Debug
+var exception = null;
+
+function breakListener(event, exec_state, event_data, data) {
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    exec_state.prepareStep(Debug.StepAction.StepIn, 1);
+    // Assert that the break happens at an intended location.
+    assertTrue(exec_state.frame(0).sourceLineText().indexOf("// break") > 0);
+  } catch (e) {
+    exception = e;
+  }
+}
+
+Debug.setListener(breakListener);
+
+debugger;                          // break
+
+function f(x) {
+  return x;                        // break
+}                                  // break
+
+Debug.setBreakPoint(f, 0, 0);      // break
+Debug.scripts();                   // break
+debug.MakeMirror(f);               // break
+
+new Error("123").stack;            // break
+Math.sin(0);                       // break
+
+f("this should break");            // break
+
+Debug.setListener(null);           // break
+
+f("this should not break");
+
+assertNull(exception);