Convert legacy 'native code' to JS-forms in dart:html

lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate

Index: lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate
diff --git a/lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate b/lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate
index f1d863063e2b64561c27c88666877cefcc404f34..886b77c92394d15b87febcad50fd433378e709cd 100644
--- a/lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate
+++ b/lib/html/templates/html/dart2js/factoryprovider_Blob.darttemplate
@@ -16,9 +16,9 @@ class $FACTORYPROVIDER {
     return _create_2(blobParts, bag);
   }
 
-  static _create_1(parts) native 'return new Blob(parts);';
-  static _create_2(parts, bag) native 'return new Blob(parts, bag);';
+  static _create_1(parts) => JS('Blob', 'new Blob(#)', parts);
+  static _create_2(parts, bag) => JS('Blob', 'new Blob(#, #)', parts, bag);
 
-  static _create_bag() native 'return {}';
-  static _bag_set(bag, key, value) native 'bag[key] = value;';
+  static _create_bag() => JS('var', '{}');
+  static _bag_set(bag, key, value) { JS('void', '#[#] = #', bag, key, value); }
 }