dart2js cps: Add helpers for common IR manipulation.

pkg/compiler/lib/src/cps_ir/redundant_phi.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.redundant_phi_elimination;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart';
 
 /// Eliminate redundant phis from the given [FunctionDefinition].
@@ skipping 169 matching lines @@
   return true;
 }
 
 /// Moves [letCont] below the binding of [definition] within the IR graph.
 /// Does nothing if [letCont] is already within the scope of [definition].
 /// Assumes that one argument is nested within the scope of the other
 /// when this method is called.
 void _moveIntoScopeOf(LetCont letCont, Definition definition) {
   if (_isInScopeOf(letCont, definition)) return;
 
-  // Remove the continuation binding from its current spot.
-  InteriorNode parent = letCont.parent;
-  parent.body = letCont.body;
-  letCont.body.parent = parent;
-
-  // Insert it just below the binding of definition.
   InteriorNode binding = definition.parent;
-
-  letCont.body = binding.body;
-  binding.body.parent = letCont;
-
-  binding.body = letCont;
-  letCont.parent = binding;
+  letCont.remove();
+  letCont.insertBelow(binding);
 }
 
 /// Ensures [continuation] has its own LetCont binding by creating
 /// a new LetCont below its current binding, if necessary.
 /// 
 /// Returns the LetCont that now binds [continuation].
 LetCont _makeUniqueBinding(Continuation continuation) {
   LetCont letCont = continuation.parent;
   if (letCont.continuations.length == 1) return letCont;
   letCont.continuations.remove(continuation);
-  LetCont newBinding = new LetCont(continuation, letCont.body);
-  newBinding.body.parent = newBinding;
-  newBinding.parent = letCont;
-  letCont.body = newBinding;
+  LetCont newBinding = new LetCont(continuation, null);
   continuation.parent = newBinding;
+  newBinding.insertBelow(letCont);
   return newBinding;
 }