Make Scanner::kNoSourcePos be a true sentinel value

runtime/observatory/tests/service/async_next_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';
 
 foo() async { }
 
 doAsync(stop) async {
   if (stop) debugger();
   await foo(); // Line 14.
   await foo(); // Line 15.
   await foo(); // Line 16.
   return null;
 }
 
 testMain() {
   // With two runs of doAsync floating around, async step should only cause
   // us to stop in the run we started in.
   doAsync(false);
   doAsync(true);
 }
 
 asyncNext(Isolate isolate) async {
+  print('asyncNext');
   return isolate.asyncStepOver()[Isolate.kSecondResume];
 }
 
 var tests = [
   hasStoppedAtBreakpoint,
   stoppedAtLine(14),
   asyncNext,
   hasStoppedAtBreakpoint,
   stoppedAtLine(15),
   asyncNext,
   hasStoppedAtBreakpoint,
   stoppedAtLine(16),
   resumeIsolate,
 ];
 
 main(args) => runIsolateTests(args, tests, testeeConcurrent: testMain);