Use Uint8List for byte lists in dart:convert.

sdk/lib/convert/ascii.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.
 
 part of dart.convert;
 
 /**
  * An instance of the default implementation of the [AsciiCodec].
  *
  * This instance provides a convenient access to the most common ASCII
@@ skipping 57 matching lines @@
 }
 
 // Superclass for [AsciiEncoder] and [Latin1Encoder].
 // Generalizes common operations that only differ by a mask;
 class _UnicodeSubsetEncoder extends Converter<String, List<int>> {
   final int _subsetMask;
 
   const _UnicodeSubsetEncoder(this._subsetMask);
 
   List<int> convert(String string) {
-    // TODO(11971): Use Uint8List when possible.
-    List result = new List<int>(string.length);
+    List result = new Uint8List(string.length);
     for (int i = 0; i < string.length; i++) {
       var codeUnit = string.codeUnitAt(i);
       if ((codeUnit & ~_subsetMask) != 0) {
         throw new ArgumentError("String contains invalid characters.");
       }
       result[i] = codeUnit;
     }
     return result;
   }
 
@@ skipping 210 matching lines @@
     }
     if (start < end) {
       if (start != 0 || end != length) {
         source = source.sublist(start, end);
       }
       add(source);
     }
     if (isLast) close();
   }
 }