Fix a crash in Chain.terse and Chain.foldFrames.

test/chain/chain_test.dart

 // Copyright (c) 2015, 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 'dart:async';
 
 import 'package:path/path.dart' as p;
 import 'package:stack_trace/stack_trace.dart';
 import 'package:test/test.dart';
 
@@ skipping 91 matching lines @@
             'package:stack_trace/stack_trace.dart 10:11  Bar.baz\n'
             'dart:core 10:11                             Zip.zap'),
         new Trace.parse(
             'dart:core 10:11                             A.b\n'
             'package:stack_trace/stack_trace.dart 10:11  C.d\n'
             'dart:core 10:11                             E.f')
       ]);
 
       expect(chain.terse.toString(), equals('dart:core  E.f\n'));
     });
+
+    // Regression test for #9
+    test("doesn't crash on empty traces", () {
+      var chain = new Chain([
+        new Trace.parse('user/code.dart 10:11  Bang.qux'),
+        new Trace([]),
+        new Trace.parse('user/code.dart 10:11  Bang.qux')
+      ]);
+
+      expect(chain.terse.toString(), equals(
+          '$userSlashCode 10:11  Bang.qux\n'
+          '===== asynchronous gap ===========================\n'
+          '$userSlashCode 10:11  Bang.qux\n'));
+    });
   });
 
   group('Chain.foldFrames', () {
     test('folds each trace', () {
       var chain = new Chain([
         new Trace.parse(
             'a.dart 10:11  Foo.bar\n'
             'a.dart 10:11  Bar.baz\n'
             'b.dart 10:11  Bang.qux\n'
             'a.dart 10:11  Zip.zap\n'
@@ skipping 151 matching lines @@
         controller = new StreamController()..addError('error', trace);
         expect(Chain.track(controller.stream).toList()
             .catchError((e, stackTrace) {
           expect(e, equals('error'));
           expect(stackTrace.toString(), equals(trace.toString()));
         }), completes);
       });
     });
   });
 }