[sat fixit] Don't call setState for unmounted components

Summary:
Before this change, in very rare cases, `component.setState()` was
being called on components that weren't actually mounted. In particular, this
happened in SAT when:

 1. A component (call it `A`) was subscribed to a store using
    `StateFromStoreMixin` and triggered a flux change in that store to navigate
    to a new URL.
 2. Backbone started updating the `StateFromStoreMixin`s that were subscribed to
    that store.
 3. The parent component of `A` received the update, re-rendered, and `A`
    was no longer in the render tree, and was unmounted.
 4. Backbone continued updating the `StateFromStoreMixin` instances, eventually
    reaching `A`'s instance.
 5. `A`'s `StateFromStoreMixin` attempted to call `.setState()` to update to
    the new values, but `A` is no longer mounted.

The solution to this is for `StateFromStore` to check whether the component is
mounted before calling `.setState()`. We could do this using
`component.isMounted()` but
[`.isMounted` is an antipattern](https://facebook.github.io/react/blog/2015/12/16/ismounted-antipattern.html)
and will be deprecated. Instead, we manually keep track of an `isMounted`
value (as suggested by the article) and check that.

Test Plan:
 - Visit http://localhost:8081/mission/sat/practice/math
 - Open a practice task
 - See no warning about `setState()` being called on an unmounted component.

Reviewers: amy, mattdunnrankin

Reviewed By: amy

Subscribers: lauren, #gtp

Differential Revision: https://phabricator.khanacademy.org/D34577

Author: xymostech

js/state-from-store-mixin.jsx

@@ -43,11 +43,14 @@
 
 const StateFromStore = function(stateDescriptors) {
     const storageKey = "StateFromStoreMixin" + (new Date).getTime();
+    let isMounted = false;
 
     const setState = function(component, stateKey, stateData) {
-        const newState = {};
-        newState[stateKey] = stateData;
-        component.setState(newState);
+        if (isMounted) {
+            const newState = {};
+            newState[stateKey] = stateData;
+            component.setState(newState);
+        }
     };
 
     /**
@@ -155,11 +158,13 @@ const StateFromStore = function(stateDescriptors) {
         },
 
         componentDidMount: function() {
+            isMounted = true;
             addChangeListeners(this);
         },
 
         componentWillUnmount: function() {
             removeChangeListeners(this);
+            isMounted = false;
         },
 
         componentWillReceiveProps: function(nextProps) {