Make _Utf8Encoder create its buffer through an external function.

runtime/lib/convert_patch.dart

Index: runtime/lib/convert_patch.dart
diff --git a/runtime/lib/convert_patch.dart b/runtime/lib/convert_patch.dart
index 3ba4f7e470dba38067ac3241ec0692d959f0c244..93da46140cb3fac8b3477af430d4d278bd61a3d0 100644
--- a/runtime/lib/convert_patch.dart
+++ b/runtime/lib/convert_patch.dart
@@ -2,6 +2,8 @@
 // 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 "dart:typed_data";
+
 // JSON conversion.
 
 patch _parseJson(String json, reviver(var key, var value)) {
@@ -554,3 +556,9 @@ class _JsonParser {
     throw new FormatException("Unexpected character at $position: $slice");
   }
 }
+
+// UTF-8 conversion.
+
+patch class _Utf8Encoder {
+  /* patch */ static List<int> _createBuffer(int size) => new Uint8List(size);
+}