async: Write BackoffMachine's methods properly.

As Greg points out [1], about the current (subtly incorrect) form:

"""
I believe the semantics of this is actually that that
[`methodName = …`] becomes part of the constructor! Not a method on
the class.
"""

We want methods on the class, so, write them that way (as "method
definitions") [2].

In particular, in an upcoming commit where we test a new `tryFetch`
implementation that retries a network request until a timeout, we'll
want to mock `BackoffMachine.prototype.wait` so we don't have to
think about it within those tests. (BackoffMachine has its own unit
tests, after all.) Making `wait` an actual method on the class lets
us accomplish this straightforwardly.

[1] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/user_avatar_url_field_optional/near/909006
[2] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Method_definitions

src/utils/async.js

@@ -38,7 +38,9 @@ export class BackoffMachine {
    * Use this to implement "give up" logic by breaking out of the loop after a
    * threshold number of waits.
    */
-  waitsCompleted = (): number => this._waitsCompleted;
+  waitsCompleted(): number {
+    return this._waitsCompleted;
+  }
 
   /**
    * Promise to resolve after the appropriate duration.
@@ -58,7 +60,7 @@ export class BackoffMachine {
    * a capped exponential shape on the expected value. Greg discusses this in more
    * detail in #3841.
    */
-  wait = async (): Promise<void> => {
+  async wait(): Promise<void> {
     if (this._startTime === undefined) {
       this._startTime = Date.now();
     }
@@ -74,7 +76,7 @@ export class BackoffMachine {
     await sleep(duration);
 
     this._waitsCompleted++;
-  };
+  }
 }
 
 /**