Clean up and simplify WebSocket implementation on the JS side

Summary:- Get rid of no longer necessary WebSocket.js v WebSocketBase.js split
- Use `EventTarget(list, of, events)` as base class to auto-generate `oneventname` getters/setters that get invoked along with other event handlers
- Type annotation `any` considered harmful, especially when we can easily spell out the actual type
- Throw in some `const` goodness for free

**Test Plan:** Launch UIExplorer example app, supplied `websocket_test_server` script, and try different combinations of sending and receiving text and binary data on both iOS and Android.
Closes https://github.com/facebook/react-native/pull/6889

Differential Revision: D3184835

Pulled By: mkonicek

fb-gh-sync-id: f21707f4e97aa5a79847f5157e0a9f132a1a01cd
fbshipit-source-id: f21707f4e97aa5a79847f5157e0a9f132a1a01cd

Examples/UIExplorer/websocket_test_server.js

@@ -0,0 +1,43 @@
+/**
+ * The examples provided by Facebook are for non-commercial testing and
+ * evaluation purposes only.
+ *
+ * Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NON INFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ *
+ * @flow
+ */
+'use strict';
+
+/* eslint-env node */
+
+const WebSocket = require('ws');
+
+console.log(`\
+Test server for WebSocketExample
+
+This will send each incoming message right back to the other side.a
+Restart with the '--binary' command line flag to have it respond with an
+ArrayBuffer instead of a string.
+`);
+
+const respondWithBinary = process.argv.indexOf('--binary') !== -1;
+const server = new WebSocket.Server({port: 5555});
+server.on('connection', (ws) => {
+  ws.on('message', (message) => {
+    console.log('Received message: %s', message);
+    if (respondWithBinary) {
+      message = new Buffer(message);
+    }
+    ws.send(message);
+  });
+
+  console.log('Incoming connection!');
+  ws.send('Why hello there!');
+});