Make new StreamController be async by default.

tests/standalone/io/string_decoder_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import "package:expect/expect.dart";
 import "dart:async";
 import "dart:io";
 
 void test() {
   // Code point U+10FFFF is the largest code point supported by Dart.
-  var controller = new StreamController();
+  var controller = new StreamController(sync: true);
   controller.add([0xf0, 0x90, 0x80, 0x80]);  // U+10000
   controller.add([0xf4, 0x8f, 0xbf, 0xbf]);  // U+10FFFF
   controller.add([0xf4, 0x90, 0x80, 0x80]);  // U+110000
   controller.add([0xfa, 0x80, 0x80, 0x80, 0x80]);  //  U+2000000
   controller.add([0xfd, 0x80, 0x80, 0x80, 0x80, 0x80]);  // U+40000000
   controller.close();
 
   var decoder = new StringDecoder(Encoding.UTF_8, '?'.codeUnitAt(0));
   var stream = controller.stream.transform(decoder);
   stream.fold(
@@ skipping 13 matching lines @@
         Expect.equals(0xdfff, decoded.codeUnitAt(3));
         Expect.equals(replacementChar, decoded.codeUnitAt(4));
         Expect.equals(replacementChar, decoded.codeUnitAt(5));
         Expect.equals(replacementChar, decoded.codeUnitAt(6));
         Expect.equals(replacementChar, decoded.codeUnitAt(7));
       });
 }
 
 void testInvalid() {
   void invalid(var bytes, var outputLength) {
-    var controller = new StreamController();
+    var controller = new StreamController(sync: true);
     controller.add(bytes);
     controller.close();
     controller.stream.transform(new StringDecoder()).listen((string) {
       Expect.equals(outputLength, string.length);
       for (var i = 0; i < outputLength; i++) {
         Expect.equals(0xFFFD, string.codeUnitAt(i));
       }
     });
   }
 
   invalid([0x80], 1);
   invalid([0xff], 1);
   invalid([0xf0, 0xc0], 1);
   invalid([0xc0, 0x80], 1);
   invalid([0xfd, 0x80, 0x80], 3); // Unfinished encoding.
 }
 
 void main() {
   test();
   testInvalid();
 }