Prefer const wherever possible

Summary:
Non-final identifiers make code harder to understand.
This is particularly true for JavaScript where even the *type*
can change as a value gets reassigned later.

This enforces to use `const` whereever possible, but doesn't
"outlaw" `let`. Mixed destructuring is also still allowed.

Used `eslint --fix` to change all existing cases.

Reviewed By: jknoxville

Differential Revision: D16131329

fbshipit-source-id: 2eceaca7c603b71b36e005be5d135e1849f2518d

src/reducers/connections.js

@@ -143,7 +143,7 @@ const reducer = (state: State = INITAL_STATE, action: Action): State => {
         selectedPlugin: DEFAULT_PLUGIN,
       };
 
-      let canBeDefaultDevice = !DEFAULT_DEVICE_BLACKLIST.some(
+      const canBeDefaultDevice = !DEFAULT_DEVICE_BLACKLIST.some(
         blacklistedDevice => payload instanceof blacklistedDevice,
       );
 
@@ -272,7 +272,7 @@ const reducer = (state: State = INITAL_STATE, action: Action): State => {
     case 'CLIENT_REMOVED': {
       const {payload} = action;
 
-      let selected = {};
+      const selected = {};
       if (state.selectedApp === payload) {
         selected.selectedApp = null;
         selected.selectedPlugin = DEFAULT_PLUGIN;