Add call tree filter to Observatory CPU profile tree

runtime/observatory/lib/src/cpu_profile/cpu_profile.dart

Index: runtime/observatory/lib/src/cpu_profile/cpu_profile.dart
diff --git a/runtime/observatory/lib/src/cpu_profile/cpu_profile.dart b/runtime/observatory/lib/src/cpu_profile/cpu_profile.dart
index 75b693bbdcbc49d411ed6a777a01fff17d4cbdc3..fb63fba880fbe476f25d4804a84d2485a11bc4fe 100644
--- a/runtime/observatory/lib/src/cpu_profile/cpu_profile.dart
+++ b/runtime/observatory/lib/src/cpu_profile/cpu_profile.dart
@@ -300,8 +300,8 @@ class _FilteredCodeCallTreeBuilder extends _FilteredCallTreeBuilder {
                 new CodeCallTreeNode(tree.root.profileData,
                                      tree.root.count)));
 
-  _copyNode(FunctionCallTreeNode node) {
-    return new FunctionCallTreeNode(node.profileData, node.count);
+  _copyNode(CodeCallTreeNode node) {
+    return new CodeCallTreeNode(node.profileData, node.count);
   }
 }