Use Uint8List for byte lists in dart:convert.

sdk/lib/convert/utf.dart

Index: sdk/lib/convert/utf.dart
diff --git a/sdk/lib/convert/utf.dart b/sdk/lib/convert/utf.dart
index 9c416261b6accc52346485b15814a86d17152a0a..332598759af6b4276d9e36506699df1f656df2ae 100644
--- a/sdk/lib/convert/utf.dart
+++ b/sdk/lib/convert/utf.dart
@@ -134,11 +134,10 @@ class _Utf8Encoder {
   _Utf8Encoder.withBufferSize(int bufferSize)
       : _buffer = _createBuffer(bufferSize);
 
-  // TODO(11971): Always use Uint8List.
   /**
    * Allow an implementation to pick the most efficient way of storing bytes.
    */
-  external static List<int> _createBuffer(int size);
+  static List<int> _createBuffer(int size) => new Uint8List(size);
 
   /**
    * Tries to combine the given [leadingSurrogate] with the [nextCodeUnit] and