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/chrome/LocationsButton.js

@@ -29,7 +29,7 @@ class LocationsButton extends Component<Props> {
   goToLocation = (location: string) => {
     const {selectedDevice} = this.props;
     if (selectedDevice instanceof AndroidDevice) {
-      let shellCommand = `am start ${location}`;
+      const shellCommand = `am start ${location}`;
       selectedDevice.adb.shell(selectedDevice.serial, shellCommand);
     }
   };

src/chrome/PluginDebugger.js

@@ -161,7 +161,7 @@ class PluginDebugger extends Component<Props> {
   }
 
   getRows(): Array<TableBodyRow> {
-    let rows: Array<TableBodyRow> = [];
+    const rows: Array<TableBodyRow> = [];
 
     // bundled plugins are loaded from the defaultPlugins directory within
     // Flipper's package.