Add support for fields to the new dart backend.

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

 library tree_ir.optimization;
 
 import '../tree_ir_nodes.dart';
 import '../../elements/elements.dart';
 import '../../constants/values.dart' as values;
 
 part 'copy_propagator.dart';
 part 'logical_rewriter.dart';
 part 'loop_rewriter.dart';
 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);
+  }
+}