Improvements to causal async stack traces

tests/language/vm/causal_async_exception_stack_test.dart

 // Copyright (c) 2016, 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';
 }
 
 number() async {
   return 4;
 }
 
 generator() async* {
   yield await number();
   yield await thrower();
 }
 
 foo() async {
   await for (var i in generator()) {
     print(i);
   }
 }
 
 main() async {
   try {
     await foo();
   } catch (e, st) {
     expect(st.toString(), stringContainsInOrder([

[rmacnak, 2017/02/10 21:27:14]

Since we're futzing with qualified names, add also a test that includes a local
function in the stack. Something like

main() {
  inner() async {
     await thrower();
  }
  inner();
}

should give

thrower
<gap>
main.inner
<gap>
main

[Cutch, 2017/02/11 00:02:40]

Done.

-        'thrower.<thrower_async_body>',
-        '<asynchronous suspension>',
         'thrower',
-        'generator.<generator_async_gen_body>',
         '<asynchronous suspension>',
         'generator',
-        'foo.<foo_async_body>',
         '<asynchronous suspension>',
+        'foo',
+        '<asynchronous suspension>',
+        'main',
         ]));
   }
 }