Overhaul tree IR visitor and rename IR classes.

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;
 import 'variable_merger.dart';
 
 export 'variable_merger.dart' show VariableMerger;
 
 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(ExecutableDefinition root) => root.applyPass(this);
-  void rewriteFieldDefinition(FieldDefinition root);
-  void rewriteFunctionDefinition(FunctionDefinition root);
-  void rewriteConstructorDefinition(ConstructorDefinition root);
+  void rewrite(RootNode root);
 
   String get passName;
 }
-
-
-abstract class PassMixin implements Pass {
-  void rewrite(ExecutableDefinition root) => root.applyPass(this);
-  void rewriteExecutableDefinition(ExecutableDefinition root);
-  void rewriteFieldDefinition(FieldDefinition root) {
-    if (!root.hasInitializer) return;
-    rewriteExecutableDefinition(root);
-  }
-  void rewriteFunctionDefinition(FunctionDefinition root) {
-    if (root.isAbstract) return;
-    rewriteExecutableDefinition(root);
-  }
-  void rewriteConstructorDefinition(ConstructorDefinition root) {
-    if (root.isAbstract) return;
-    rewriteExecutableDefinition(root);
-  }
-}
-