Add support for fields to the new dart backend.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
index 40e47b3dfe2452b410dc7b60efd5713588ecca68..121cccc2c8c92739d199f02c173c7619b29a0004 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
@@ -43,6 +43,14 @@ class SExpressionStringifier extends Visitor<String> with Indentation {
            '$body)';
   }
 
+  String visitFieldDefinition(FieldDefinition node) {
+    String name = node.element.name;
+    namer.useReturnName(node.returnContinuation);
+    String body = indentBlock(() => visit(node.body));
+    return '$indentation(FieldDefinition $name (return)\n'
+           '$body)';
+  }
+
   String visitLetPrim(LetPrim node) {
     String name = newValueName(node.primitive);
     String value = visit(node.primitive);