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

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

Index: pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
diff --git a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
index e91d58e64a5fde9fe1caa1a52b58b4e7000ae546..c3823ece560f55358510d2e298239fc3fc9ffc72 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -486,6 +486,7 @@ class _RemovalVisitor extends RecursiveVisitor {
 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;