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 c4621cb605b85f18245a5e028eacd95ae3172c4e..2756afe7936c4f2fb6e7a0bd88ea133c0a09bcbd 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
@@ -32,6 +32,14 @@ class SExpressionStringifier extends Visitor<String> with Indentation {
            '$body)';
   }
 
+  String visitFieldDefinition(FieldDefinition node) {
+    String name = node.element.name;
+    names[node.returnContinuation] = 'return';
+    String body = indentBlock(() => visit(node.body));
+    return '$indentation(FieldDefinition $name (return)\n'
+           '$body)';
+  }
+
   String visitLetPrim(LetPrim node) {
     String name = newValueName();
     names[node.primitive] = name;