Add tests of line numbers to causal async stack tests

tests/language/vm/causal_async_exception_stack_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.
 
 import 'package:unittest/unittest.dart';
 
 thrower() async {
   throw 'oops';
 }
 
@@ skipping 11 matching lines @@
     print(i);
   }
 }
 
 main() async {
   // Test async and async*.
   try {
     await foo();
   } catch (e, st) {
     expect(st.toString(), stringContainsInOrder([
-        'thrower',
+        'thrower', '.dart:8',
         '<asynchronous suspension>',
-        'generator',
+        'generator', '.dart:17',
         '<asynchronous suspension>',
-        'foo',
+        'foo', '.dart:21',
         '<asynchronous suspension>',
         'main',
         ]));
   }
 
   inner() async {
     deep() async {
       await thrower();
     }
     await deep();
@@ skipping 13 matching lines @@
           'main',
           '<asynchronous suspension>',
           ]));
   }
 
   // Test for correct linkage.
   try {
     await thrower();
   } catch(e, st) {
     expect(st.toString(), stringContainsInOrder([
-           'thrower',
+           'thrower', '.dart:8',
            '<asynchronous suspension>',
-           'main',
+           'main', '.dart:67',
            ]));
   }
 }