Move file utils

Fixing IC after moving it from Gradle to the compiler in commit f40a3eff20

build-common/src/org/jetbrains/kotlin/incremental/buildUtil.kt

@@ -255,8 +255,6 @@ fun <Target> mapClassesFqNamesToFiles(
     return dirtyFiles
 }
 
-private fun File.isJavaFile() = extension.equals(JavaFileType.INSTANCE.defaultExtension, ignoreCase = true)
-
 private fun findSrcDirRoot(file: File, roots: Iterable<File>): File? =
         roots.firstOrNull { FileUtil.isAncestor(it, file, false) }
 

build-common/src/org/jetbrains/kotlin/incremental/fileUtils.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2010-2016 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.kotlin.incremental
+
+import java.io.File
+
+fun File.isJavaFile() =
+        extension.equals("java", ignoreCase = true)
+
+fun File.isKotlinFile(): Boolean =
+    extension.let {
+        "kt".equals(it, ignoreCase = true) ||
+        "kts".equals(it, ignoreCase = true)
+    }
+
+fun File.isClassFile(): Boolean =
+        extension.equals("class", ignoreCase = true)