Debugger: use debug break slots to break at function exit.

test/mjsunit/debug-return-value.js

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 106 matching lines @@
         frames = response.body().frames;
         assertTrue(frames[0].atReturn);
         assertEquals(expected_return_value,
                      response.lookup(frames[0].returnValue.ref).value);
 
         listener_complete = true;
       }
     }
   } catch (e) {
     exception = e
+    print(e + e.stack)
   };
 };
 
 // Add the debug event listener.
 Debug.setListener(listener);
 
 // Four steps from the debugger statement in this function will position us at
 // the function return.
 //             0         1         2         3         4         5
 //             0123456789012345678901234567890123456789012345678901
@@ skipping 17 matching lines @@
 assertTrue(listener_complete);
 assertEquals(4, break_count);
 
 break_count = 0;
 expected_return_value = 2;
 listener_complete = false;
 f(false);
 assertFalse(exception, "exception in listener")
 assertTrue(listener_complete);
 assertEquals(4, break_count);