Make `getProperty` and `callMethod` nullable

sdk/lib/js_util/dart2js/js_util_dart2js.dart

Index: sdk/lib/js_util/dart2js/js_util_dart2js.dart
diff --git a/sdk/lib/js_util/dart2js/js_util_dart2js.dart b/sdk/lib/js_util/dart2js/js_util_dart2js.dart
index 23a46e0a16a0a5e3bfb62ece529f5687be244ec8..ec6c09a5d69bae61551c09cb27d7bb4188b81f2b 100644
--- a/sdk/lib/js_util/dart2js/js_util_dart2js.dart
+++ b/sdk/lib/js_util/dart2js/js_util_dart2js.dart
@@ -59,11 +59,11 @@ _convertDataTree(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) {