Incremental compilation of added instance fields.

dart/pkg/compiler/lib/src/elements/modelx.dart

Index: dart/pkg/compiler/lib/src/elements/modelx.dart
diff --git a/dart/pkg/compiler/lib/src/elements/modelx.dart b/dart/pkg/compiler/lib/src/elements/modelx.dart
index bf88fa763600fc2fd5c8fd557fc878cf3f56ccef..007e58482e9c7fbdbee9805c16f1c56679984a8b 100644
--- a/dart/pkg/compiler/lib/src/elements/modelx.dart
+++ b/dart/pkg/compiler/lib/src/elements/modelx.dart
@@ -1260,6 +1260,11 @@ class FieldElementX extends VariableElementX
     super.reuseElement();
     nestedClosures.clear();
   }
+
+  FieldElementX copyWithEnclosing(Element enclosingElement) {
+    return new FieldElementX(
+        new Identifier(token), enclosingElement, variables);
+  }
 }
 
 /// [Element] for a parameter-like element.