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/dispatcher/iOSDevice.js

@@ -160,7 +160,7 @@ async function checkXcodeVersionMismatch() {
     let {stdout: xcodeCLIVersion} = await exec('xcode-select -p');
     xcodeCLIVersion = xcodeCLIVersion.trim();
     const {stdout} = await exec('ps aux | grep CoreSimulator');
-    for (let line of stdout.split('\n')) {
+    for (const line of stdout.split('\n')) {
       const match = line.match(
         /\/Applications\/Xcode[^/]*\.app\/Contents\/Developer/,
       );