Overhaul tree IR visitor and rename IR classes.

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart2js.cps_ir.optimizers;
 
 /// Eliminate redundant phis from the given [FunctionDefinition].
 ///
 /// Phis in this case are [Continuations] together with corresponding
 /// [InvokeContinuation]s. A [Continuation] parameter at position i is redundant
 /// if for all [InvokeContinuation]s, the parameter at position i is identical
 /// (except for feedback). Redundant parameters are removed from the
 /// continuation signature, all invocations, and replaced within the
 /// continuation body.
-class RedundantPhiEliminator extends RecursiveVisitor with PassMixin {
+class RedundantPhiEliminator extends RecursiveVisitor implements Pass {
   String get passName => 'Redundant phi elimination';
 
   final Set<Continuation> workSet = new Set<Continuation>();
 
   @override
-  void rewriteExecutableDefinition(final ExecutableDefinition root) {
+  void rewrite(RootNode root) {
+    if (root.isEmpty) return;
 
     // Set all parent pointers.
     new ParentVisitor().visit(root);
 
     // Traverse the tree once to build the work set.
     visit(root);
 
     // Process each continuation one-by-one.
     while (workSet.isNotEmpty) {
       Continuation cont = workSet.first;
@@ skipping 160 matching lines @@
 
   // Insert it just below the binding of definition.
   InteriorNode binding = definition.parent;
 
   letCont.body = binding.body;
   binding.body.parent = letCont;
 
   binding.body = letCont;
   letCont.parent = binding;
 }