Add tests of line numbers to causal async stack tests

runtime/observatory/tests/service/causal_async_star_stack_contents_test.dart

 // Copyright (c) 2017, 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 'dart:developer';
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'service_test_common.dart';
@@ skipping 57 matching lines @@
   (Isolate isolate) async {
     ServiceMap stack = await isolate.getStack();
     // Has causal frames (we are inside a function called by an async function)
     expect(stack['asyncCausalFrames'], isNotNull);
     var asyncStack = stack['asyncCausalFrames'];
     expect(asyncStack[0].toString(), contains('foobar'));
     expect(asyncStack[1].kind, equals(M.FrameKind.asyncSuspensionMarker));
     expect(asyncStack[2].toString(), contains('helper'));
     expect(asyncStack[3].kind, equals(M.FrameKind.asyncSuspensionMarker));
     expect(asyncStack[4].toString(), contains('testMain'));
+    // Line 21.
+    expect(await asyncStack[0].location.toUserString(), contains(':21:'));
+    // Line 27.
+    expect(await asyncStack[2].location.toUserString(), contains(':27:'));
+    // Line 30.
+    expect(await asyncStack[4].location.toUserString(), contains(':33:'));
   },
 ];
 
 main(args) => runIsolateTestsSynchronous(args,
                                          tests,
                                          testeeConcurrent: testMain);