allow JS builtin to be typed as needed

lib/runtime/dart/html.js

 dart_library.library('dart/html', null, /* Imports */[
   'dart/_runtime',
   'dart/math',
   'dart/core',
   'dart/_js_helper',
   'dart/collection',
   'dart/async',
   'dart/_foreign_helper',
   'dart/isolate'
 ], /* Lazy imports */[
@@ skipping 10047 matching lines @@
               exports._ElementCssClassSet._classListRemove(list, item);
             } else {
               i = dart.notNull(i) + 1;
             }
           }
         }
         static _classListOf(e) {
           return dart.as(wrap_jso(e.raw.classList), DomTokenList);
         }
         static _classListLength(list) {
-          return dart.as(list.raw.length, core.int);
+          return list.raw.length;
         }
         static _classListContains(list, value) {
-          return dart.as(list.raw.contains(value), core.bool);
+          return list.raw.contains(value);
         }
         static _classListContainsBeforeAddOrRemove(list, value) {
-          return dart.as(list.raw.contains(value), core.bool);
+          return list.raw.contains(value);
         }
         static _classListAdd(list, value) {
           list.raw.add(value);
         }
         static _classListRemove(list, value) {
           list.raw.remove(value);
         }
         static _classListToggle1(list, value) {
           return list.raw.toggle(value);
         }
@@ skipping 2715 matching lines @@
   exports.ElementUpgrader = ElementUpgrader;
   exports.NodeValidator = NodeValidator;
   exports.NodeTreeSanitizer = NodeTreeSanitizer;
   exports.UriPolicy = UriPolicy;
   exports.spawnDomUri = spawnDomUri;
   exports.unwrap_jso = unwrap_jso;
   exports.wrap_jso = wrap_jso;
   exports.createCustomUpgrader = createCustomUpgrader;
   exports.getHtmlCreateFunction = getHtmlCreateFunction;
 });