Refactor DartTypeVisitor and ElementVisitor.

pkg/compiler/lib/src/scanner/class_element_parser.dart

Index: pkg/compiler/lib/src/scanner/class_element_parser.dart
diff --git a/pkg/compiler/lib/src/scanner/class_element_parser.dart b/pkg/compiler/lib/src/scanner/class_element_parser.dart
index 27c7b9ff4278c921319648e5f60b0315962e7c61..9bf1523feb9eb33ad2dcc7d8336fc32c806dffcf 100644
--- a/pkg/compiler/lib/src/scanner/class_element_parser.dart
+++ b/pkg/compiler/lib/src/scanner/class_element_parser.dart
@@ -91,7 +91,9 @@ class PartialClassElement extends ClassElementX with PartialElement {
   Modifiers get modifiers =>
       cachedNode != null ? cachedNode.modifiers : Modifiers.EMPTY;
 
-  accept(ElementVisitor visitor) => visitor.visitClassElement(this);
+  accept(ElementVisitor visitor, arg) {
+    return visitor.visitClassElement(this, arg);
+  }
 
   PartialClassElement copyWithEnclosing(CompilationUnitElement enclosing) {
     return new PartialClassElement(name, beginToken, endToken, enclosing, id);