Add support for fields to the new dart backend.

pkg/compiler/lib/src/tree_ir/optimization/optimization.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart b/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
index d7b8e086f8354a892319df91717a78efc7bdf0b0..63652c45e3f0e1810d6a0b9a9148811b999706c5 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
@@ -12,5 +12,20 @@ part 'statement_rewriter.dart';
 /// An optimization pass over the Tree IR.
 abstract class Pass {
   /// Applies optimizations to root, rewriting it in the process.
-  void rewrite(FunctionDefinition root);
+  void rewrite(ExecutableDefinition root) => root.applyPass(this);
+  void rewriteFieldDefinition(FieldDefinition root);
+  void rewriteFunctionDefinition(FunctionDefinition root);
 }
+
+
+abstract class PassMixin implements Pass {
+  void rewrite(ExecutableDefinition root) => root.applyPass(this);
+  void rewriteExecutableDefinition(ExecutableDefinition root);
+  void rewriteFieldDefinition(FieldDefinition root) {
+    rewriteExecutableDefinition(root);
+  }
+  void rewriteFunctionDefinition(FunctionDefinition root) {
+    if (root.isAbstract) return;
+    rewriteExecutableDefinition(root);
+  }
+}