Improvement on future: add stack trace to future failures.

lib/core/future.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.
 
 /**
  * A [Future] is used to obtain a value sometime in the future.  Receivers of a
  * [Future] can obtain the value by passing a callback to [then]. For example:
  *
  *     Future<int> future = getFutureFromSomewhere();
  *     future.then((value) {
@@ skipping 175 matching lines @@
 
 /**
  * Thrown if a completer tries to set the value on a future that is already
  * complete.
  */
 class FutureAlreadyCompleteException implements Exception {
   FutureAlreadyCompleteException() {}
   String toString() => "Exception: future already completed";
 }
 
+/**
+ * Wraps unhandled exceptions provided to [Completer.completeException]. It is
+ * used to show both the error message and the stack trace for unhandled
+ * exceptions.
+ */
+class FutureUnhandledException implements Exception {
+  /** Wrapped exception. */
+  var source;
+
+  /** Trace for the wrapped exception. */
+  Object stackTrace;
+
+  FutureUnhandledException(this.source, this.stackTrace);
+
+  String toString() {
+    return 'FutureUnhandledException: exception while executing Future\n  '
+        '${source.toString().replaceAll("\n", "\n  ")}\n'
+        'original stack trace:\n  '
+        '${stackTrace.toString().replaceAll("\n","\n  ")}';
+  }
+}
+
 
 /**
  * [Futures] holds additional utility functions that operate on [Future]s (for
  * example, waiting for a collection of Futures to complete).
  */
 class Futures {
   /**
    * Returns a future which will complete once all the futures in a list are
    * complete. If any of the futures in the list completes with an exception,
    * the resulting future also completes with an exception. (The value of the
@@ skipping 25 matching lines @@
       future.handleException((exception) {
         if (!result.isComplete) {
           completer.completeException(exception, future.stackTrace);
         }
         return true;
       });
     }
     return result;
   }
 }