Implement shrinking reductions in CPS IR

sdk/lib/_internal/compiler/implementation/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.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 {
+class RedundantPhiEliminator extends RecursiveVisitor implements Pass {
   final Map<Continuation, List<InvokeContinuation>> cont2invokes =
       <Continuation, List<InvokeContinuation>>{};
   // For each reference r used in a continuation invocation i, stores the
   // corresponding continuation i.continuation. If required by other passes,
   // we could consider adding parent pointers to references instead.
   final Map<Reference, Continuation> ref2cont = <Reference, Continuation>{};
   final Set<Continuation> workSet = new Set<Continuation>();
 
   void rewrite(final FunctionDefinition root) {
     // Traverse the tree once to build the work set.
@@ skipping 99 matching lines @@
         .add(node);
 
     // And the reference map.
     node.arguments.forEach((Reference ref) {
       assert(!ref2cont.containsKey(ref));
       ref2cont[ref] = node.continuation.definition;
     });
   }
 }