Fix bad onSuccess calls after an exception.

sdk/lib/async/stream_pipe.dart

Index: sdk/lib/async/stream_pipe.dart
diff --git a/sdk/lib/async/stream_pipe.dart b/sdk/lib/async/stream_pipe.dart
index f3a8ea119ecbfdeb6d8294034ffc0648c835f7da..aa3da2a664aebfb6506a985d95b7aa040c2c234c 100644
--- a/sdk/lib/async/stream_pipe.dart
+++ b/sdk/lib/async/stream_pipe.dart
@@ -25,6 +25,8 @@ _runUserCode(userCode(), onSuccess(value), onError(AsyncError error),
     } else {
       onError(new AsyncError.withCause(e, s, cause));
     }
+    // onError is allowed to return. Don't execute the onSuccess below.
+    return;
   }
   onSuccess(result);
 }