Remove AsyncError with Expando.

sdk/lib/io/io_sink.dart

Index: sdk/lib/io/io_sink.dart
diff --git a/sdk/lib/io/io_sink.dart b/sdk/lib/io/io_sink.dart
index 0f955b329be0033dbf869f900e7500471626a5dc..8001905509676c265355189d1ecebed38dac85ef 100644
--- a/sdk/lib/io/io_sink.dart
+++ b/sdk/lib/io/io_sink.dart
@@ -33,7 +33,7 @@ abstract class IOSink implements StreamSink<List<int>>, StringSink {
   /**
    * Writes an error to the consumer.
    */
-  void addError(AsyncError error);
+  void addError(error);
 
   /**
    * Adds all elements of the given [stream] to `this`.
@@ -123,7 +123,7 @@ class _IOSinkImpl implements IOSink {
     _controller.add(data);
   }
 
-  void addError(AsyncError error) {
+  void addError(error) {
     _controller.addError(error);
   }