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..cecad3191c8bbc8f295d11f3cc67b0f8b5d9a3c6 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_fragment.dart
@@ -83,6 +83,7 @@ class CpsFragment {
     }
     if (context != null) {
       context.body = node;
+      node.parent = context;
     }
     context = null;
   }
@@ -297,4 +298,19 @@ 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);
+  }
 }