dart2js cps: Maintain parent pointers instead of recomputing them.

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

Index: pkg/compiler/lib/src/cps_ir/cps_fragment.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
index 4a1f34056156fdb184f01b250f4e70d3856e78cf..79ad510e3957e6608e47f1ffdbdafbf50c607a97 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -10,6 +10,7 @@ import '../universe/selector.dart' show Selector;
 import '../types/types.dart' show TypeMask;
 import '../io/source_information.dart';
 import '../elements/elements.dart';
+import 'parent_visitor.dart';
 
 /// Builds a CPS fragment that can be plugged into another CPS term.
 ///
@@ -83,6 +84,7 @@ class CpsFragment {
     }
     if (context != null) {
       context.body = node;
+      node.parent = context;
     }
     context = null;
   }
@@ -297,4 +299,19 @@ class CpsFragment {
     put(let);
     context = let;
   }
+
+  void insertBelow(InteriorNode node) {
+    assert(isOpen);
+    if (isEmpty) return;
+    Expression child = node.body;
+    node.body = root;
+    root.parent = node;
+    context.body = child;
+    child.parent = context;
+    root = context = null;
+  }
+
+  void insertAbove(InteriorExpression node) {
+    insertBelow(node.parent);
+  }
 }