Add support for fields to the new dart backend.

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

Index: pkg/compiler/lib/src/cps_ir/optimizers.dart
diff --git a/pkg/compiler/lib/src/cps_ir/optimizers.dart b/pkg/compiler/lib/src/cps_ir/optimizers.dart
index c64650fa611a4159297449a239e093e160bce596..de19a0dd7ccb8c14ccd597e36d700c0c1d0b1f81 100644
--- a/pkg/compiler/lib/src/cps_ir/optimizers.dart
+++ b/pkg/compiler/lib/src/cps_ir/optimizers.dart
@@ -21,5 +21,11 @@ part 'shrinking_reductions.dart';
 /// An optimization pass over the CPS IR.
 abstract class Pass {
   /// Applies optimizations to root, rewriting it in the process.
-  void rewrite(FunctionDefinition root);
+  void rewrite(ExecutableDefinition root) => root.acceptPass(this);
+  void rewriteFunctionDefinition(FunctionDefinition root) {
+    return root.acceptPass(this);
+  }
+  void rewriteFieldDefinition(FieldDefinition root) {
+    return root.acceptPass(this);
+  }
 }