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';
 }
 
@@ skipping 10 matching lines @@
   await for (var i in generator()) {
     print(i);
   }
 }
 
 main() async {
   try {
     await foo();
   } catch (e, st) {
     expect(st.toString(), stringContainsInOrder([
-        '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',
         ]));

[siva, 2017/02/11 00:24:33]

might be interesting to add
      try {
         await thrower();
      } catch(e, st) {
         expect(st.toString(), stringContainsInOrder([
           'thrower',
           '<asynchronous suspension>',
           'main',
         ]));
      }

This checks that the linking works and we don't retain any of the
previous links (foo, generator etc.)

[Cutch, 2017/02/11 01:14:28]

Done.

   }
+
+  inner() async {
+    deep() async {
+      await thrower();
+    }
+    await deep();
+  }
+
+  try {
+    await inner();
+  } catch (e, st) {
+    expect(st.toString(), stringContainsInOrder([
+          'thrower',
+          '<asynchronous suspension>',
+          'main.inner.deep',
+          '<asynchronous suspension>',
+          'main.inner',
+          '<asynchronous suspension>',
+          'main',
+          '<asynchronous suspension>',
+          ]));
+  }
 }