reorg: move JS ops implementations to cli/js/ops/, part 3 (#4302)

Following JS ops were moved to separate files in cli/js/ops directory:
- net
- tls
- fs

cli/js/ops/fs/seek.ts

@@ -0,0 +1,33 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+import { sendSync, sendAsync } from "../dispatch_json.ts";
+import { SeekMode } from "../../io.ts";
+
+/** Synchronously seek a file ID to the given offset under mode given by `whence`.
+ *
+ * Returns the number of cursor position.
+ *
+ *       const file = Deno.openSync("/foo/bar.txt");
+ *       const position = Deno.seekSync(file.rid, 0, 0);
+ */
+export function seekSync(
+  rid: number,
+  offset: number,
+  whence: SeekMode
+): number {
+  return sendSync("op_seek", { rid, offset, whence });
+}
+
+/** Seek a file ID to the given offset under mode given by `whence`.
+ *
+ * Resolves with the number of cursor position.
+ *
+ *      const file = await Deno.open("/foo/bar.txt");
+ *      const position = await Deno.seek(file.rid, 0, 0);
+ */
+export async function seek(
+  rid: number,
+  offset: number,
+  whence: SeekMode
+): Promise<number> {
+  return await sendAsync("op_seek", { rid, offset, whence });
+}