Break apart the File class into multiple files and add type definitions.

packages/babel-core/src/transform.js

@@ -0,0 +1,13 @@
+// @flow
+import loadConfig from "./config";
+import runTransform, { type FileResult } from "./transformation";
+
+export default function transform(
+  code: string,
+  opts?: Object,
+): FileResult | null {
+  const config = loadConfig(opts);
+  if (config === null) return null;
+
+  return runTransform(config, code);
+}