Revert "Use an explicit 'this' parameter instead of 'This' nodes."

pkg/compiler/lib/src/cps_ir/shrinking_reductions.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.
 
 part of dart2js.cps_ir.optimizers;
 
 /**
  * [ShrinkingReducer] applies shrinking reductions to CPS terms as described
  * in 'Compiling with Continuations, Continued' by Andrew Kennedy.
  */
@@ skipping 468 matching lines @@
         }
       }
     }
   }
 }
 
 /// Traverses the CPS term and sets node.parent for each visited node.
 class ParentVisitor extends RecursiveVisitor {
   processFunctionDefinition(FunctionDefinition node) {
     node.body.parent = node;
-    if (node.thisParameter != null) node.thisParameter.parent = node;
     int index = 0;
     node.parameters.forEach((Definition parameter) {
       parameter.parent = node;
       if (parameter is Parameter) parameter.parentIndex = index++;
     });
   }
 
   processRunnableBody(RunnableBody node) {
     node.returnContinuation.parent = node;
     node.body.parent = node;
@@ skipping 209 matching lines @@
   }
 
   String toString() => "$kind: $node";
 }
 
 /// A dummy class used solely to mark nodes as deleted once they are removed
 /// from a term.
 class _DeletedNode extends Node {
   accept(_) => null;
 }