Add Accessor, Getter, and Setter elements.

pkg/compiler/lib/src/dart_backend/placeholder_collector.dart

Index: pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
diff --git a/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart b/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
index 91ed14927e147a1efa69cfdcb9dfc7171db7cdd6..04a7b7c9263413866084fc7ef5fd9d266d45dd2f 100644
--- a/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
+++ b/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
@@ -369,8 +369,8 @@ class PlaceholderCollector extends Visitor {
       return;
     }
 
-    if (element.isGetter || element.isSetter) {
-      element = (element as FunctionElement).abstractField;
+    if (element.isAccessor) {
+      element = (element as AccessorElement).abstractField;
     }
     elementNodes.putIfAbsent(element, () => new Set<Node>()).add(node);
   }