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/plugins/fresco/index.js

@@ -302,7 +302,7 @@ export default class extends FlipperPlugin<PluginState, *, PersistedState> {
       this.props.setPersistedState({imagesMap: images}),
     );
 
-    let images = this.filterImages(
+    const images = this.filterImages(
       this.props.persistedState.images,
       this.props.persistedState.events,
       this.state.selectedSurface,