Don't use FileUtil in BuiltInsSerializer

See the comment

core/util.runtime/src/org/jetbrains/jet/utils/fileUtils.kt

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2010-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.jet.utils
+
+import java.io.File
+
+// Similar to kotlin.io.recurse, but processes each directory after its children
+public fun File.recursePostOrder(block: (File) -> Unit): Unit {
+    val children = this.listFiles()
+    if (children != null) {
+        for (child in children) {
+            child.recursePostOrder(block)
+        }
+    }
+    block(this)
+}