Cleanup of various DOM dart2js and dart_analyzer warnings

tools/dom/src/dart2js_Conversions.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 
 // Conversions for Window.  These check if the window is the local
 // window, and if it's not, wraps or unwraps it with a secure wrapper.
 // We need to test for EventTarget here as well as it's a base type.
 // We omit an unwrapper for Window as no methods take a non-local
 // window as a parameter.
@@ skipping 14 matching lines @@
   return _DOMWindowCrossFrame._createSafe(win);
 }
 
 EventTarget _convertNativeToDart_EventTarget(e) {
   if (e == null) {
     return null;
   }
   // Assume it's a Window if it contains the setInterval property.  It may be
   // from a different frame - without a patched prototype - so we cannot
   // rely on Dart type checking.
-  if (JS('bool', r'"setInterval" in #', e))
-    return _DOMWindowCrossFrame._createSafe(e);
+  if (JS('bool', r'"setInterval" in #', e)) {
+    var window = _DOMWindowCrossFrame._createSafe(e);
+    // If it's a native window.
+    if (window is EventTarget) {
+      return window;
+    }
+    return null;
+  }
   else
     return e;
 }
 
 EventTarget _convertDartToNative_EventTarget(e) {
   if (e is _DOMWindowCrossFrame) {
     return e._window;
   } else {
     return e;
   }
@@ skipping 34 matching lines @@
 
 // We can get rid of this conversion if _TypedImageData implements the fields
 // with native names.
 _convertDartToNative_ImageData(ImageData imageData) {
   if (imageData is _TypedImageData) {
     return JS('', '{data: #, height: #, width: #}',
         imageData.data, imageData.height, imageData.width);
   }
   return imageData;
 }