Make `getProperty` and `callMethod` nullable

sdk/lib/js_util/dart2js/js_util_dart2js.dart

 // Copyright (c) 2016, 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.
 
 /// Utility methods to efficiently manipulate typed JSInterop objects in cases
 /// where the name to call is not known at runtime. You should only use these
 /// methods when the same effect cannot be achieved with @JS annotations.
 /// These methods would be extension methods on JSObject if Dart supported
 /// extension methods.
 library dart.js_util;
@@ skipping 41 matching lines @@
       return o;
     }
   }
 
   return _convert(data);
 }
 
 newObject() => JS('=Object', '{}');
 
 hasProperty(o, name) => JS('bool', '# in #', name, o);
-getProperty(o, name) => JS('Object', '#[#]', o, name);
+getProperty(o, name) => JS('Object|Null', '#[#]', o, name);
 setProperty(o, name, value) => JS('', '#[#]=#', o, name, value);
 
 callMethod(o, String method, List args) =>
-    JS('Object', '#[#].apply(#, #)', o, method, o, args);
+    JS('Object|Null', '#[#].apply(#, #)', o, method, o, args);
 
 instanceof(o, Function type) => JS('bool', '# instanceof #', o, type);
 callConstructor(Function constr, List arguments) {
   if (arguments == null) {
     return JS('Object', 'new #()', constr);
   }
 
   if (JS('bool', '# instanceof Array', arguments)) {
     int argumentCount = JS('int', '#.length', arguments);
     switch (argumentCount) {
@@ skipping 40 matching lines @@
   // object for which there is an interceptor
   return JS('Object', 'new #()', factoryFunction);
 
   // TODO(sra): Investigate:
   //
   //     var jsObj = JS('', 'Object.create(#.prototype)', constr);
   //     JS('', '#.apply(#, #)', constr, jsObj,
   //         []..addAll(arguments.map(_convertToJS)));
   //     return _wrapToDart(jsObj);
 }