Remove as checks bloating sample dart apps by 4K

tools/dom/src/shared_html.dart

Index: tools/dom/src/shared_html.dart
diff --git a/tools/dom/src/shared_html.dart b/tools/dom/src/shared_html.dart
index 27fa2b705f99e634a264b4c1db9e7389ae3794eb..7342cdfef31238ae884a8abcffebc73faef68749 100644
--- a/tools/dom/src/shared_html.dart
+++ b/tools/dom/src/shared_html.dart
@@ -14,7 +14,10 @@ _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) {
@@ -22,7 +25,10 @@ _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;
 }
 
 /**