Make _Utf8Encoder create its buffer through an external function.

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 8e480f80333904569054ec0557754049fac40049..2f513b05ccc8fcabf11e53e2f22177a4fe4883ce 100644
--- a/sdk/lib/convert/utf.dart
+++ b/sdk/lib/convert/utf.dart
@@ -133,8 +133,13 @@ class _Utf8Encoder {
   _Utf8Encoder() : this.withBufferSize(_DEFAULT_BYTE_BUFFER_SIZE);
 
   _Utf8Encoder.withBufferSize(int bufferSize)
-      // TODO(11971, floitsch): use Uint8List instead of normal lists.
-      : _buffer = new List<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);
 
   /**
    * Tries to combine the given [leadingSurrogate] with the [nextCodeUnit] and