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.

3 files changed, 36 insertions, 10 deletions

diff --git a/js/globals.ts b/js/globals.ts
index f17d2f33a..a6f1b0927 100644
--- a/js/globals.ts
+++ b/js/globals.ts
@@ -3,6 +3,7 @@
 import { Console } from "./console";
 import { RawSourceMap } from "./types";
 import * as timers from "./timers";
+import { TextEncoder, TextDecoder } from "./text_encoding";
 
 declare global {
   interface Window {
@@ -16,6 +17,11 @@ declare global {
 
   const console: Console;
   const window: Window;
+
+  // tslint:disable:variable-name
+  let TextEncoder: TextEncoder;
+  let TextDecoder: TextDecoder;
+  // tslint:enable:variable-name
 }
 
 // If you use the eval function indirectly, by invoking it via a reference
@@ -49,10 +55,8 @@ window.clearTimeout = timers.clearTimer;
 window.clearInterval = timers.clearTimer;
 
 window.console = new Console(libdeno.print);
+window.TextEncoder = TextEncoder;
+window.TextDecoder = TextDecoder;
 
 // import { fetch } from "./fetch";
 // window["fetch"] = fetch;
-
-// import { TextEncoder, TextDecoder } from "text-encoding";
-// window["TextEncoder"] = TextEncoder;
-// window["TextDecoder"] = TextDecoder;
diff --git a/js/text-encoding.d.ts b/js/text-encoding.d.ts
deleted file mode 100644
index 6feadad9c..000000000
--- a/js/text-encoding.d.ts
+++ /dev/null
@@ -1,6 +0,0 @@
-// Remove and depend on @types/text-encoding once this PR is merged
-// https://github.com/DefinitelyTyped/DefinitelyTyped/pull/26141
-declare module "text-encoding" {
-  export const TextEncoder: TextEncoder;
-  export const TextDecoder: TextDecoder;
-}
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";