Remove as checks bloating sample dart apps by 4K

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 1d4407bd3319bce10102472c62b0664454219acb..5c18688f2a13551437962f39ff15f5122bbe26c8 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -43390,7 +43390,10 @@ _wrapZoneCallback/*<A, R>*/ _wrapZone/*<A, R>*/(_wrapZoneCallback/*<A, R>*/ call
   if (callback == null) return null;
   // TODO(jacobr): we cast to _wrapZoneCallback/*<A, R>*/ to hack around missing
   // generic method support in zones.
-  return Zone.current.bindUnaryCallback(callback, runGuarded: true) as _wrapZoneCallback/*<A, R>*/;
+  // ignore: STRONG_MODE_DOWN_CAST_COMPOSITE
+  _wrapZoneCallback/*<A, R>*/ wrapped =
+      Zone.current.bindUnaryCallback(callback, runGuarded: true);
+  return wrapped;
 }
 
 _wrapZoneBinaryCallback/*<A, B, R>*/ _wrapBinaryZone/*<A, B, R>*/(_wrapZoneBinaryCallback/*<A, B, R>*/ callback) {
@@ -43398,7 +43401,10 @@ _wrapZoneBinaryCallback/*<A, B, R>*/ _wrapBinaryZone/*<A, B, R>*/(_wrapZoneBinar
   if (callback == null) return null;
   // We cast to _wrapZoneBinaryCallback/*<A, B, R>*/ to hack around missing
   // generic method support in zones.
-  return Zone.current.bindBinaryCallback(callback, runGuarded: true) as _wrapZoneBinaryCallback/*<A, B, R>*/;
+  // ignore: STRONG_MODE_DOWN_CAST_COMPOSITE
+  _wrapZoneBinaryCallback/*<A, B, R>*/ wrapped =
+      Zone.current.bindBinaryCallback(callback, runGuarded: true);
+  return wrapped;
 }
 
 /**