dart2dart Fix collecting type of class fields

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index 5a9e0e770c95710c926f309d4e193e0ef1aea53e..91112f586e5081d1c45e1e1bb5aa93f21cccffe4 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -145,8 +145,10 @@ class ReferencedElementCollector extends AbstractVisitor {
 
   ReferencedElementCollector(
       this.compiler,
-      this.rootElement, this.treeElements,
-      this.newTypedefElementCallback, this.newClassElementCallback);
+      Element rootElement, this.treeElements,
+      this.newTypedefElementCallback, this.newClassElementCallback)
+      : this.rootElement = (rootElement is VariableElement)
+          ? (rootElement as VariableElement).variables : rootElement;
 
   visitClassNode(ClassNode node) {
     super.visitClassNode(node);