Support tab completion of line:col in the debugger.

runtime/observatory/tests/service/vm_restart_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=--compile-all --error_on_bad_type --error_on_bad_override
 
 import 'dart:async';
 import 'dart:developer';
-import 'dart:io';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 int count = 0;
 
 void test() {
   while (true) {
     count++;
     debugger();
@@ skipping 78 matching lines @@
     print("restarting");
     await isolate.vm.restart();
     await restartCompleter.future;
     print("restarted");
     await testCompleter.future;
   },
 ];
 
   
 main(args) => runIsolateTests(args, tests, testeeConcurrent: test);