allow JS builtin to be typed as needed

lib/runtime/dart/html_common.js

Index: lib/runtime/dart/html_common.js
diff --git a/lib/runtime/dart/html_common.js b/lib/runtime/dart/html_common.js
index 0f24eba8b9f1355839dc3f43f2cc5e4dfed88382..cf35432025cb2bb56cf07baa6d0826ed7c367675 100644
--- a/lib/runtime/dart/html_common.js
+++ b/lib/runtime/dart/html_common.js
@@ -448,7 +448,7 @@ dart_library.library('dart/html_common', null, /* Imports */[
     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 {
@@ -473,7 +473,7 @@ dart_library.library('dart/html_common', null, /* Imports */[
       }
       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) {
@@ -570,7 +570,7 @@ dart_library.library('dart/html_common', null, /* Imports */[
       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]);
       }
     }