dart2js cps: Add helpers for common IR manipulation.

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 747e701a26112abc500069caa1f4cd3e3ea04f28..d07d5190ab480754da3971396e74e6d0b89a72d5 100644
--- a/pkg/compiler/lib/src/cps_ir/redundant_join.dart
+++ b/pkg/compiler/lib/src/cps_ir/redundant_join.dart
@@ -64,20 +64,6 @@ class RedundantJoinEliminator extends RecursiveVisitor implements Pass {
     }
   }
 
-  /// Removes [movedNode] from its current position and inserts it
-  /// before [target].
-  void moveToBefore(Expression target, LetCont movedNode) {
-    if (movedNode.parent != null) {
-      movedNode.parent.body = movedNode.body;
-      movedNode.body.parent = movedNode.parent;
-    }
-    InteriorNode parent = target.parent;
-    parent.body = movedNode;
-    movedNode.body = target;
-    target.parent = movedNode;
-    movedNode.parent = parent;
-  }
-
   void rewriteBranch(Branch branch) {
     InteriorNode parent = getEffectiveParent(branch);
     if (parent is! Continuation) return;
@@ -161,7 +147,8 @@ class RedundantJoinEliminator extends RecursiveVisitor implements Pass {
           }
         }
       }
-      moveToBefore(outerLetCont, innerLetCont);
+      innerLetCont.remove();
+      innerLetCont.insertAbove(outerLetCont);
     }
 
     assert(branchCont.body == branch);
@@ -196,9 +183,7 @@ class RedundantJoinEliminator extends RecursiveVisitor implements Pass {
     branch.falseContinuation.unlink();
     outerLetCont.continuations.remove(branchCont);
     if (outerLetCont.continuations.isEmpty) {
-      InteriorNode parent = outerLetCont.parent;
-      parent.body = outerLetCont.body;
-      outerLetCont.body.parent = parent;
+      outerLetCont.remove();
     }
 
     // We may have created new redundant join points in the two branches.