Properly handle empty stack traces in package:stack_trace.

pkg/stack_trace/test/trace_test.dart

 // Copyright (c) 2013, 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.
 
 library trace_test;
 
 import 'package:pathos/path.dart' as path;
 import 'package:stack_trace/stack_trace.dart';
 import 'package:unittest/unittest.dart';
 
@@ skipping 84 matching lines @@
 
       expect(trace.frames[0].uri,
           equals(Uri.parse("http://pub.dartlang.org/thing.js")));
       expect(trace.frames[1].uri,
           equals(Uri.parse("http://pub.dartlang.org/stuff.js")));
       expect(trace.frames[2].uri,
           equals(Uri.parse("http://pub.dartlang.org/stuff.js")));
     });
 
     test('parses an empty string correctly', () {
-      expect(new Trace.parse('').frames, isEmpty);
+      var trace = new Trace.parse('');
+      expect(trace.frames, isEmpty);
+      expect(trace.toString(), equals(''));
     });
   });
 
   test('.toString() nicely formats the stack trace', () {
     var trace = new Trace.parse('''
 #0      Foo._bar (foo/bar.dart:42:21)
 #1      zip.<anonymous closure>.zap (dart:async/future.dart:0:2)
 #2      zip.<anonymous closure>.zap (http://pub.dartlang.org/thing.dart:1:100)
 ''');
 
@@ skipping 44 matching lines @@
 
     var folded = trace.foldFrames((frame) => frame.member.startsWith('foo'));
     expect(folded.toString(), equals('''
 foo.dart 42:21                     notFoo
 foo.dart 1:100                     fooBottom
 bar.dart 10:20                     alsoNotFoo
 dart:async-patch/future.dart 9:11  fooBottom
 '''));
   });
 }