dart2js cps: Maintain parent pointers instead of recomputing them.

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

+// Copyright (c) 2015, 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 cps_ir.parent_visitor;
+
+import 'cps_ir_nodes.dart';
+
+/// Traverses the CPS term and sets node.parent for each visited node.
+class ParentVisitor extends DeepRecursiveVisitor {
+  static void setParents(Node node) {
+    ParentVisitor visitor = new ParentVisitor._make();
+    visitor._worklist.add(node);
+    visitor.trampoline();
+  }
+
+  /// Private to avoid accidental `new ParentVisitor().visit(node)` calls.
+  ParentVisitor._make();
+
+  Node _parent;
+  final List<Node> _worklist = <Node>[];
+
+  void trampoline() {
+    while (_worklist.isNotEmpty) {
+      _parent = _worklist.removeLast();
+      _parent.accept(this);
+    }
+  }
+
+  @override
+  visit(Node node) {
+    _worklist.add(node);
+    assert(_parent != node);
+    assert(_parent != null);
+    node.parent = _parent;
+  }
+
+  @override
+  processReference(Reference node) {
+    node.parent = _parent;
+  }
+}
+