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

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

Index: pkg/compiler/lib/src/cps_ir/redundant_join.dart
diff --git a/pkg/compiler/lib/src/cps_ir/redundant_join.dart b/pkg/compiler/lib/src/cps_ir/redundant_join.dart
index 2a1a584c99417c824a70033fd911bdd285fb6eac..4acb2ef438b834dbe6a7924d8c9a6c895ecd679f 100644
--- a/pkg/compiler/lib/src/cps_ir/redundant_join.dart
+++ b/pkg/compiler/lib/src/cps_ir/redundant_join.dart
@@ -94,8 +94,7 @@ class RedundantJoinEliminator extends RecursiveVisitor implements Pass {
     // 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;