Add error-intercept for Completer.completeError and StreamController.addError.

sdk/lib/async/async_error.dart

Index: sdk/lib/async/async_error.dart
diff --git a/sdk/lib/async/async_error.dart b/sdk/lib/async/async_error.dart
index cac1e4ddcf64878b830f985f3c80634f1fd0cb0d..9a640859b1247dd668c995fc5fa788bea8453703 100644
--- a/sdk/lib/async/async_error.dart
+++ b/sdk/lib/async/async_error.dart
@@ -21,14 +21,7 @@ Function _registerErrorHandler(Function errorHandler, Zone zone) {
   }
 }
 
-class _AsyncError implements Error {
-  final error;
-  final StackTrace stackTrace;
-
-  _AsyncError(this.error, this.stackTrace);
-}
-
-class _UncaughtAsyncError extends _AsyncError {
+class _UncaughtAsyncError extends AsyncError {
   _UncaughtAsyncError(error, StackTrace stackTrace)
       : super(error, _getBestStackTrace(error, stackTrace));