Print the stack traces for Future expectations that raise exceptions.

lib/unittest/future_matchers.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Matches a [Future] that completes successfully with a value. Note that this
  * creates an asynchronous expectation. The call to `expect()` that includes
  * this will return immediately and execution will continue. Later, when the
  * future completes, the actual expectation will run.
  *
@@ skipping 15 matching lines @@
 
 class _Completes extends BaseMatcher {
   final Matcher _matcher;
 
   const _Completes(this._matcher);
 
   bool matches(item) {
     if (item is! Future) return false;
 
     item.onComplete(expectAsync1((future) {
-      expect(future.hasValue,
-          reason: 'Expected future to complete successfully, but it failed');
+      var reason = 'Expected future to complete successfully, but it failed '
+        'with ${future.exception}';
+      if (future.stackTrace != null) {
+        var stackTrace = future.stackTrace.toString();
+        stackTrace = '  ${stackTrace.replaceAll('\n', '\n  ')}';
+        reason = '$reason\nStack trace:\n$stackTrace';
+      }
+
+      expect(future.hasValue, reason: reason);
       if (_matcher != null) expect(future.value, _matcher);
     }));
 
     return true;
   }
 
   Description describe(Description description) {
     if (_matcher == null) {
       description.add('completes successfully');
     } else {
       description.add('completes to a value that ').addDescriptionOf(_matcher);
     }
     return description;
   }
 }