Improvement on future: add stack trace to future failures.

lib/core/future.dart

Index: lib/core/future.dart
diff --git a/lib/core/future.dart b/lib/core/future.dart
index 99af2415180efcd019af18bce4ad6a7800ef087b..902432a64b90aa0a49be44957923cc0976fc79ea 100644
--- a/lib/core/future.dart
+++ b/lib/core/future.dart
@@ -193,6 +193,28 @@ class FutureAlreadyCompleteException implements Exception {
   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