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

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

 // Copyright (c) 2015, 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.redundant_join_elimination;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 
 /// Eliminates redundant join points.
@@ skipping 76 matching lines @@
     if (!branchCont.hasMultipleUses) return;
 
     // It might be beneficial to rewrite calls to recursive continuations,
     // but we currently do not support this.
     if (branchCont.isRecursive) return;
 
     // Check that the branching condition is a parameter on the
     // enclosing continuation.
     // Note: Do not use the parent pointer for this check, because parameters
     // are temporarily shared between different continuations during this pass.
-    IsTrue isTrue = branch.condition;
-    Primitive condition = isTrue.value.definition;
+    Primitive condition = branch.condition.definition;
     int parameterIndex = branchCont.parameters.indexOf(condition);
     if (parameterIndex == -1) return;
 
     // Check that all callers hit a fixed branch, and count the number
     // of times each branch is hit.
     // We know all callers are InvokeContinuations because they are the only
     // valid uses of a multi-use continuation.
     int trueHits = 0, falseHits = 0;
     InvokeContinuation trueCall, falseCall;
     for (Reference ref = branchCont.firstRef; ref != null; ref = ref.next) {
@@ skipping 157 matching lines @@
     });
   }
 
   processReference(Reference ref) {
     Parameter target = renaming[ref.definition];
     if (target != null) {
       ref.changeTo(target);
     }
   }
 }