dart2js cps: Redundant join elimination.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
index ebb8574fa66247fe85ac0eee98cb26ca0e74fa14..3cc0f217f39f822458b51a6b02d15def640bb410 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -128,6 +128,17 @@ class Reference<T extends Definition<T>> {
     }
     if (next != null) next.previous = previous;
   }
+
+  /// Changes the definition referenced by this object and updates
+  /// the reference chains accordingly.
+  void changeTo(Definition<T> newDefinition) {
+    unlink();
+    previous = null;
+    definition = newDefinition;
+    next = definition.firstRef;
+    if (next != null) next.previous = this;
+    definition.firstRef = this;
+  }
 }
 
 /// Evaluates a primitive and binds it to variable: `let val x = V in E`.