refactor: remove mock-fs from dev dependency

src/fs/node.ts

@@ -0,0 +1,22 @@
+import * as _ from '../util/underscore'
+import { resolve, extname } from 'path'
+import { stat, readFile } from 'fs'
+import IFS from './ifs'
+
+const statAsync = _.promisify(stat) as (filepath: string) => Promise<object>
+const readFileAsync = _.promisify(readFile) as (filepath: string, encoding: string) => Promise<string>
+
+const fs: IFS = {
+  exists: filepath => {
+    return statAsync(filepath).then(() => true).catch(() => false)
+  },
+  readFile: filepath => {
+    return readFileAsync(filepath, 'utf8')
+  },
+  resolve: (root: string, file: string, ext: string) => {
+    if (!extname(file)) file += ext
+    return resolve(root, file)
+  }
+}
+
+export default fs