WebSocket polyfill

Summary:
- Added as a library in /Libraries/WebSocket
- Drag and drop to add to project (similar to adding Geolocation polyfill)
- Exposed as `window.WebSocket` which conforms with https://developer.mozilla.org/en-US/docs/Web/API/WebSocket specs
Closes https://github.com/facebook/react-native/pull/890
Github Author: Harrison Harnisch <hharnisc@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/WebSocket/WebSocket.ios.js

@@ -0,0 +1,104 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule WebSocket
+ *
+ */
+'use strict';
+
+var RCTDeviceEventEmitter = require('RCTDeviceEventEmitter');
+var RCTWebSocketManager = require('NativeModules').WebSocketManager;
+
+var WebSocketBase = require('WebSocketBase');
+
+var WebSocketId = 0;
+
+class WebSocket extends WebSocketBase {
+  _socketId: number;
+  _subs: any;
+
+  connectToSocketImpl(url: string): void {
+    this._socketId = WebSocketId++;
+    RCTWebSocketManager.connect(url, this._socketId);
+    this._registerEvents(this._socketId);
+  }
+
+  closeConnectionImpl(): void {
+    RCTWebSocketManager.close(this._socketId);
+  }
+
+  cancelConnectionImpl(): void {
+    RCTWebSocketManager.close(this._socketId);
+  }
+
+  sendStringImpl(message: string): void {
+    RCTWebSocketManager.send(message, this._socketId);
+  }
+
+  sendArrayBufferImpl(): void {
+    // TODO
+    console.warn('Sending ArrayBuffers is not yet supported');
+  }
+
+  _unregisterEvents(): void {
+    this._subs.forEach(e => e.remove());
+    this._subs = [];
+  }
+
+  _registerEvents(id: number): void {
+    this._subs = [
+      RCTDeviceEventEmitter.addListener(
+        'websocketMessage',
+        function(ev) {
+          if (ev.id !== id) {
+            return;
+          }
+          this.onmessage && this.onmessage({
+            data: ev.data
+          });
+        }.bind(this)
+      ),
+      RCTDeviceEventEmitter.addListener(
+        'websocketOpen',
+        function(ev) {
+          if (ev.id !== id) {
+            return;
+          }
+          this.readyState = this.OPEN;
+          this.onopen && this.onopen();
+        }.bind(this)
+      ),
+      RCTDeviceEventEmitter.addListener(
+        'websocketClosed',
+        function(ev) {
+          if (ev.id !== id) {
+            return;
+          }
+          this.readyState = this.CLOSED;
+          this.onclose && this.onclose(ev);
+          this._unregisterEvents();
+          RCTWebSocketManager.close(id);
+        }.bind(this)
+      ),
+      RCTDeviceEventEmitter.addListener(
+        'websocketFailed',
+        function(ev) {
+          if (ev.id !== id) {
+            return;
+          }
+          this.onerror && this.onerror(new Error(ev.message));
+          this._unregisterEvents();
+          RCTWebSocketManager.close(id);
+        }.bind(this)
+      )
+    ];
+  }
+
+}
+
+module.exports = WebSocket;