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/__tests__/utils.ts

@@ -0,0 +1,44 @@
+/**
+ * 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
+ */
+
+// TODO: Share with desktop/flipper-server-core/src/comms/__tests__/utils.ts
+export class WSMessageAccumulator {
+  private messages: unknown[] = [];
+  private newMessageSubscribers: ((newMessageContent: unknown) => void)[] = [];
+
+  constructor(private readonly timeout = 1000) {}
+
+  get newMessage(): Promise<unknown> {
+    return new Promise((resolve, reject) => {
+      const timer = setTimeout(() => {
+        reject(new Error('Timeout exceeded'));
+      }, this.timeout);
+
+      this.newMessageSubscribers.push((newMessageContent: unknown) => {
+        clearTimeout(timer);
+        resolve(newMessageContent);
+      });
+
+      this.consume();
+    });
+  }
+
+  add(newMessageContent: unknown) {
+    this.messages.push(newMessageContent);
+    this.consume();
+  }
+
+  private consume() {
+    if (this.messages.length && this.newMessageSubscribers.length) {
+      const message = this.messages.shift();
+      const subscriber = this.newMessageSubscribers.shift();
+      subscriber!(message);
+    }
+  }
+}