dart2js cps: Use a strictness flag on Branch instead of rewriting.

pkg/compiler/lib/src/cps_ir/shrinking_reductions.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.
 
 library dart2js.cps_ir.shrinking_reductions;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 
 /**
@@ skipping 592 matching lines @@
 
   processContinuation(Continuation node) {
     if (node.body != null) node.body.parent = node;
     int index = 0;
     node.parameters.forEach((Parameter parameter) {
       parameter.parent = node;
       parameter.parentIndex = index++;
     });
   }
 
-  processIsTrue(IsTrue node) {
-    node.value.parent = node;
-  }
-
   processInterceptor(Interceptor node) {
     node.input.parent = node;
   }
 
   processSetField(SetField node) {
     node.object.parent = node;
     node.value.parent = node;
   }
 
   processGetField(GetField node) {
@@ skipping 109 matching lines @@
   }
 
   String toString() => "$kind: $node";
 }
 
 /// A dummy class used solely to mark nodes as deleted once they are removed
 /// from a term.
 class _DeletedNode extends Node {
   accept(_) => null;
 }