Support tab completion of line:col in the debugger.

runtime/observatory/tests/service/step_into_async_no_await_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=--error_on_bad_type --error_on_bad_override  --verbose_debug
 
-import 'package:observatory/service_io.dart';
 import 'test_helper.dart';
 import 'dart:developer';
 
 // :async_op will not be captured in this function because it never needs to
 // reschedule it.
 asyncWithoutAwait() async {
   print("asyncWithoutAwait");
 }
 
 testMain() {
   debugger();
-  asyncWithoutAwait();
+  asyncWithoutAwait();  // Line 17
 }
 
 var tests = [
   hasStoppedAtBreakpoint,
-  stoppedAtLine(18),
+  stoppedAtLine(17),
   (isolate) => isolate.stepInto(),
   hasStoppedAtBreakpoint,
   (isolate) => isolate.getStack(),  // Should not crash.
   // TODO(rmacnak): stoppedAtLine(12)
   // This doesn't happen because asyncWithoutAwait is marked undebuggable.
   // Probably needs to change to support async-step-into.
   resumeIsolate,
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);