Add atob() and btoa() (#776)

js/text_encoding_test.ts

@@ -0,0 +1,26 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+import { test, assert, assertEqual } from "./test_util.ts";
+
+test(function atobSuccess() {
+  const text = "hello world";
+  const encoded = btoa(text);
+  assertEqual(encoded, "aGVsbG8gd29ybGQ=");
+});
+
+test(function btoaSuccess() {
+  const encoded = "aGVsbG8gd29ybGQ=";
+  const decoded = atob(encoded);
+  assertEqual(decoded, "hello world");
+});
+
+test(function btoaFailed() {
+  const text = "你好";
+  let err;
+  try {
+    btoa(text);
+  } catch (e) {
+    err = e;
+  }
+  assert(!!err);
+  assertEqual(err.name, "InvalidInput");
+});