Add TextEncoder/TextDecoder support.

Fixes #470

This commit increases size:
out/release/gen/bundle/main.js      7.3M -> 7.9M
out/release/gen/bundle/main.js.map   11M -> 12M
out/release/gen/snapshot_deno.bin    34M -> 37M
out/release/deno                     49M -> 53M

Note the amount in the JS code added is quite small:
4.0K    node_modules/text-encoding/index.js
4.0K    node_modules/@types/text-encoding/index.d.ts
4.0K    js/text_encoding.ts

Unclear to me what is causing the jump in snapshot size.

1 files changed, 28 insertions, 0 deletions

diff --git a/js/text_encoding.ts b/js/text_encoding.ts
new file mode 100644
index 000000000..c8e262f5e
--- /dev/null
+++ b/js/text_encoding.ts
@@ -0,0 +1,28 @@
+// Copyright 2018 the Deno authors. All rights reserved. MIT license.
+
+// @types/text-encoding relies on lib.dom.d.ts for some interfaces. We do not
+// want to include lib.dom.d.ts (due to size) into deno's global type scope.
+// Therefore this hack: add a few of the missing interfaces in
+// @types/text-encoding to the global scope before importing.
+
+declare global {
+  type BufferSource = ArrayBufferView | ArrayBuffer;
+
+  interface TextDecodeOptions {
+    stream?: boolean;
+  }
+
+  interface TextDecoderOptions {
+    fatal?: boolean;
+    ignoreBOM?: boolean;
+  }
+
+  interface TextDecoder {
+    readonly encoding: string;
+    readonly fatal: boolean;
+    readonly ignoreBOM: boolean;
+    decode(input?: BufferSource, options?: TextDecodeOptions): string;
+  }
+}
+
+export { TextEncoder, TextDecoder } from "text-encoding";