Implement JS flipper client

Summary:
Standardize WS implementation for JS environments.

Why do we need a separate server implementation for browsers?
Browser targets cannot authenticate via the default certificate exchange flow. We need a dedicated client for them that works over an insecure channel (without the cert exchange).

Major changes:
1. Renamed `flipper-js-client-sdk` to `js-flipper` for consistency with `react-native-flipper`
2. Updated `js-flipper` implementation to match our other existing clients

Documentation will be updated in a separate subsequent PR.

https://fb.quip.com/2mboA0xbgoxl

Reviewed By: mweststrate

Differential Revision: D31688105

fbshipit-source-id: 418aa80e0fd86361c089cf54b0d44a8b4f748efa

js/js-flipper/src/util.ts

@@ -0,0 +1,70 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+// https://github.com/microsoft/TypeScript/issues/36931#issuecomment-846131999
+type Assert = (condition: unknown) => asserts condition;
+export const assert: Assert = (condition) => {
+  if (!condition) {
+    throw new Error();
+  }
+};
+
+export const safeJSONStringify = (data: unknown): string => {
+  try {
+    return JSON.stringify(data);
+  } catch {
+    return 'Unable to serialize';
+  }
+};
+
+export const isPromise = (val: unknown): val is Promise<unknown> =>
+  typeof val === 'object' &&
+  val !== null &&
+  typeof (val as Promise<unknown>).then === 'function' &&
+  typeof (val as Promise<unknown>).catch === 'function';
+
+// TODO: Share types wiht desktop
+type OS =
+  | 'iOS'
+  | 'Android'
+  | 'Metro'
+  | 'Windows'
+  | 'MacOS'
+  | 'Browser'
+  | 'Linux';
+
+// https://stackoverflow.com/a/31456668
+const detectDeviceType = () =>
+  typeof process === 'object' &&
+  typeof process.versions === 'object' &&
+  typeof process.versions.node !== 'undefined'
+    ? 'Node.js'
+    : 'Browser';
+export const detectOS = (): OS => {
+  if (detectDeviceType() === 'Browser') {
+    return 'Browser';
+  }
+  switch (require('os').type()) {
+    case 'Linux':
+      return 'Linux';
+    case 'Darwin':
+      return 'MacOS';
+    default:
+      return 'Windows';
+  }
+};
+
+export const detectDevice = (): string => {
+  if (detectDeviceType() === 'Browser') {
+    return window.navigator.userAgent;
+  }
+  return require('os').release();
+};
+
+export const awaitTimeout = (timeout: number) => new Promise(resolve => setTimeout)