Fix bug in processing of latent breakpoints

tests/standalone/debugger/deferred_code_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Test to verify that a breakpoint can be set in code that is not yet
 // loaded, i.e. in a deferred library.
 //
 // This test forks a second vm process that runs this dart script as
 // a debug target.
 // Run this test with option --wire to see the json messages sent
@@ skipping 21 matching lines @@
 // Expected debugger events and commands.
 var testScript = [
   MatchFrame(0, "main"),  // Top frame in trace is function "main".
   SetBreakpoint(22),  // Breakpoint just before call to loadLibrary().
   Resume(),
   // Set BP in deferred library code before the loadLibrary() call is executed.
   MatchFrame(0, "main"),
   MatchLine(22),
   MatchLocals({"loaded": "false"}),
   SetBreakpoint(10, url: "deferred_code_lib.dart"),
+  // Regression test: we used to have a bug that hung the debugger when
+  // processing latent brakepoints for urls that have no match.
+  // The BP below will not match any loaded file.
+  SetBreakpoint(10, url: "non_existing_file.dart"),
   Resume(),
   MatchFrame(0, "stopTheBuck"),  // Expect to be stopped in deferred library code.
   // MatchLine(10), // Line matching only works for the main script.
   Resume(),
 ];