Properly handle empty stack traces in package:stack_trace.

pkg/stack_trace/lib/src/trace.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;
 
 import 'dart:collection';
 import 'dart:math' as math;
 
 import 'frame.dart';
@@ skipping 125 matching lines @@
         newFrames.add(new Frame(
             frame.uri, frame.line, frame.column, frame.member));
       }
     }
 
     return new Trace(newFrames.reversed);
   }
 
   /// Returns a human-readable string representation of [this].
   String toString() {
-    if (frames.length == '') return '';
-
     // Figure out the longest path so we know how much to pad.
-    var longest = frames.map((frame) => frame.location.length).reduce(math.max);
+    var longest = frames.map((frame) => frame.location.length)
+        .fold(0, math.max);
 
     // Print out the stack trace nicely formatted.
     return frames.map((frame) {
       return '${_padRight(frame.location, longest)}  ${frame.member}\n';
     }).join();
   }
 }
 
 /// Returns [string] with enough spaces added to the end to make it [length]
 /// characters long.
 String _padRight(String string, int length) {
   if (string.length >= length) return string;
 
   var result = new StringBuffer();
   result.write(string);
   for (var i = 0; i < length - string.length; i++) {
     result.write(' ');
   }
 
   return result.toString();
 }