Add call tree filter to Observatory CPU profile tree

runtime/observatory/lib/src/cpu_profile/cpu_profile.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.
 
 part of cpu_profiler;
 
 abstract class CallTreeNode {
   final List<CallTreeNode> children;
   final int count;
   double get percentage => _percentage;
@@ skipping 282 matching lines @@
 }
 
 class _FilteredCodeCallTreeBuilder extends _FilteredCallTreeBuilder {
   _FilteredCodeCallTreeBuilder(CallTreeNodeFilter filter,
                                CodeCallTree tree)
     : super(filter, tree,
             new CodeCallTree(tree.inclusive,
                 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);
   }
 }
 
 class FunctionCallTree extends CallTree {
   FunctionCallTree(bool inclusive, FunctionCallTreeNode root)
       : super(inclusive, root) {
     _setFunctionPercentage(null, root);
   }
 
   FunctionCallTree filtered(CallTreeNodeFilter filter) {
@@ skipping 605 matching lines @@
   int approximateMillisecondsForCount(count) {
     var MICROSECONDS_PER_MILLISECOND = 1000.0;
     return (count * samplePeriod) ~/ MICROSECONDS_PER_MILLISECOND;
   }
 
   double approximateSecondsForCount(count) {
     var MICROSECONDS_PER_SECOND = 1000000.0;
     return (count * samplePeriod) / MICROSECONDS_PER_SECOND;
   }
 }