Check for null when invoking a native method.

frog/leg/native_emitter.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.
 
 class NativeEmitter {
 
   Compiler compiler;
   bool addedDynamicFunction = false;
   bool addedTypeNameOfFunction = false;
   bool addedDefPropFunction = false;
@@ skipping 138 matching lines @@
     }
     method = method || methods.Object;
 
     if (method == null) {
       method = function() {
         throw $capture(new $exception(obj, name, arguments));
       };
     }
 
     var proto = Object.getPrototypeOf(obj);
+    var nullCheckMethod = function() {
+      var res = method.apply(this, Array.prototype.slice.call(arguments));
+      return res === null ? (void 0) : res;
+    }
     if (!proto.hasOwnProperty(name)) {
-      $defPropName(proto, name, method);
+      $defPropName(proto, name, nullCheckMethod);
     }
 
-    return method.apply(this, Array.prototype.slice.call(arguments));
+    return nullCheckMethod.apply(this, Array.prototype.slice.call(arguments));
   };
   dynamicBind.methods = methods;
   $defPropName(Object.prototype, name, dynamicBind);
   return methods;
 }''';
 }
 
   String buildDynamicMetadataCode() => '''
 if (typeof $dynamicMetadataName == 'undefined') $dynamicMetadataName = [];''';
 
@@ skipping 345 matching lines @@
         entries.add("\n    ['$clsName', ${tagDefns[cls]}]");
       }
       buffer.add(Strings.join(entries, ','));
       buffer.add('];\n');
       buffer.add('$dynamicSetMetadataName(table);\n');
 
       buffer.add('})();\n');
     }
   }
 }