Make JSON.fuse(UTF8) be more efficient by not creating intermediate string.

sdk/lib/_internal/compiler/js_lib/convert_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/convert_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/convert_patch.dart b/sdk/lib/_internal/compiler/js_lib/convert_patch.dart
index 2686ca6cdb3eb8f14436833e9244979775e88d02..bd8816f9f4a3675d2f154927a72084ed023c7900 100644
--- a/sdk/lib/_internal/compiler/js_lib/convert_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/convert_patch.dart
@@ -369,3 +369,10 @@ class _JsonDecoderSink extends _StringSinkConversionSink {
     _sink.close();
   }
 }
+
+@patch class Utf8Decoder {
+  @patch
+  Converter<List<int>,dynamic> fuse(Converter<String, dynamic> next) {
+    return super.fuse(next);
+  }
+}