Unify dart:async and pub/io.dart TimeoutException classes.

sdk/lib/async/future_impl.dart

Index: sdk/lib/async/future_impl.dart
diff --git a/sdk/lib/async/future_impl.dart b/sdk/lib/async/future_impl.dart
index fb1280c52c1b5c540057cdab773e4dd29870f84b..85ed798a7f79788b91a04436bee79f57e9077566 100644
--- a/sdk/lib/async/future_impl.dart
+++ b/sdk/lib/async/future_impl.dart
@@ -544,7 +544,8 @@ class _Future<T> implements Future<T> {
     Timer timer;
     if (onTimeout == null) {
       timer = new Timer(timeLimit, () {
-        result._completeError(new TimeoutException(timeLimit));
+        result._completeError(new TimeoutException("Future not completed",
+                                                   timeLimit));
       });
     } else {
       Zone zone = Zone.current;