allow JS builtin to be typed as needed

lib/runtime/dart/html_common.js

 dart_library.library('dart/html_common', null, /* Imports */[
   'dart/_runtime',
   'dart/_metadata',
   'dart/core',
   'dart/html',
   'dart/typed_data',
   'dart/_js_helper',
   'dart/_native_typed_data',
   'dart/async',
   'dart/collection',
@@ skipping 430 matching lines @@
       this.stencil = stencil;
     }
   }
   dart.setSignature(_TypedContextAttributes, {
     constructors: () => ({_TypedContextAttributes: [_TypedContextAttributes, [core.bool, core.bool, core.bool, core.bool, core.bool, core.bool, core.bool]]})
   });
   function convertNativeToDart_ContextAttributes(nativeContextAttributes) {
     if (dart.is(nativeContextAttributes, dart.dynamic)) {
       return nativeContextAttributes;
     }
-    return new _TypedContextAttributes(dart.as(nativeContextAttributes.alpha, core.bool), dart.as(nativeContextAttributes.antialias, core.bool), dart.as(nativeContextAttributes.depth, core.bool), dart.as(nativeContextAttributes.failIfMajorPerformanceCaveat, core.bool), dart.as(nativeContextAttributes.premultipliedAlpha, core.bool), dart.as(nativeContextAttributes.preserveDrawingBuffer, core.bool), dart.as(nativeContextAttributes.stencil, core.bool));
+    return new _TypedContextAttributes(nativeContextAttributes.alpha, nativeContextAttributes.antialias, nativeContextAttributes.depth, nativeContextAttributes.failIfMajorPerformanceCaveat, nativeContextAttributes.premultipliedAlpha, nativeContextAttributes.preserveDrawingBuffer, nativeContextAttributes.stencil);
   }
   dart.fn(convertNativeToDart_ContextAttributes);
   class _TypedImageData extends core.Object {
     _TypedImageData(data, height, width) {
       this.data = data;
       this.height = height;
       this.width = width;
     }
   }
   dart.setSignature(_TypedImageData, {
     constructors: () => ({_TypedImageData: [_TypedImageData, [typed_data.Uint8ClampedList, core.int, core.int]]})
   });
   function convertNativeToDart_ImageData(nativeImageData) {
     0;
     if (dart.is(nativeImageData, dart.dynamic)) {
       let data = dart.dload(nativeImageData, 'data');
       if (data.constructor === Array) {
         if (typeof CanvasPixelArray !== "undefined") {
           data.constructor = CanvasPixelArray;
           data.BYTES_PER_ELEMENT = 1;
         }
       }
       return nativeImageData;
     }
-    return new _TypedImageData(dart.as(nativeImageData.data, typed_data.Uint8ClampedList), dart.as(nativeImageData.height, core.int), dart.as(nativeImageData.width, core.int));
+    return new _TypedImageData(nativeImageData.data, nativeImageData.height, nativeImageData.width);
   }
   dart.fn(convertNativeToDart_ImageData);
   function convertDartToNative_ImageData(imageData) {
     if (dart.is(imageData, _TypedImageData)) {
       return {data: dart.dload(imageData, 'data'), height: dart.dload(imageData, 'height'), width: dart.dload(imageData, 'width')};
     }
     return imageData;
   }
   dart.fn(convertDartToNative_ImageData);
   const _serializedScriptValue = 'num|String|bool|' + 'JSExtendableArray|=Object|' + 'Blob|File|NativeByteBuffer|NativeTypedData';
@@ skipping 76 matching lines @@
     newJsList(length) {
       return new Array(length);
     }
     newDartList(length) {
       return this.newJsList(length);
     }
     identicalInJs(a, b) {
       return core.identical(a, b);
     }
     forEachJsField(object, action) {
-      for (let key of dart.as(Object.keys(object), core.Iterable)) {
+      for (let key of Object.keys(object)) {
         dart.dcall(action, key, object[key]);
       }
     }
   }
   dart.setSignature(_AcceptStructuredCloneDart2Js, {
     methods: () => ({
       newJsList: [dart.dynamic, [dart.dynamic]],
       newDartList: [dart.dynamic, [dart.dynamic]],
       identicalInJs: [core.bool, [dart.dynamic, dart.dynamic]],
       forEachJsField: [dart.void, [dart.dynamic, dart.dynamic]]
@@ skipping 337 matching lines @@
   exports.isJavaScriptArray = isJavaScriptArray;
   exports.isJavaScriptSimpleObject = isJavaScriptSimpleObject;
   exports.isImmutableJavaScriptArray = isImmutableJavaScriptArray;
   exports.isJavaScriptPromise = isJavaScriptPromise;
   exports.convertNativePromiseToDartFuture = convertNativePromiseToDartFuture;
   exports.Device = Device;
   exports.FilteredElementList = FilteredElementList;
   exports.Lists = Lists;
   exports.NodeListWrapper = NodeListWrapper;
 });