Make _Utf8Encoder create its buffer through an external function.

sdk/lib/_internal/lib/convert_patch.dart

Index: sdk/lib/_internal/lib/convert_patch.dart
diff --git a/sdk/lib/_internal/lib/convert_patch.dart b/sdk/lib/_internal/lib/convert_patch.dart
index 1f0b0575c8acfb97b0412b7c787de6ed1ec3b9c1..70b4d5f1babe3ac52754fff79f600289dd7bebdf 100644
--- a/sdk/lib/_internal/lib/convert_patch.dart
+++ b/sdk/lib/_internal/lib/convert_patch.dart
@@ -92,3 +92,8 @@ _convertJsonToDart(json, reviver(key, value)) {
 
   return revive(null, walk(json));
 }
+
+patch class _Utf8Encoder {
+  // Use Uint8List when supported on all platforms.
+  patch static List<int> _createBuffer(int size) => new List<int>(size);
+}