Add browser versions of the transform files.

packages/babel-core/src/transform-file-browser.js

@@ -0,0 +1,14 @@
+// @flow
+import type { FileResult } from "./transformation";
+
+export default function transformFile(
+  filename: string,
+  opts?: Object = {},
+  callback: (?Error, FileResult | null) => void,
+) {
+  if (typeof opts === "function") {
+    callback = opts;
+  }
+
+  callback(new Error("Transforming files is not supported in browsers"), null);
+}

packages/babel-core/src/transform-file-sync-browser.js

@@ -0,0 +1,5 @@
+// @flow
+
+export default function transformFileSync() {
+  throw new Error("Transforming files is not supported in browsers");
+}