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/network/__tests__/requestToCurlCommand.node.js

@@ -129,7 +129,7 @@ test('convertRequestToCurlCommand: malicious POST data', () => {
 });
 
 test('convertRequestToCurlCommand: control characters', () => {
-  let request: Request = {
+  const request: Request = {
     id: 'request id',
     timestamp: 1234567890,
     method: 'GET',
@@ -138,7 +138,7 @@ test('convertRequestToCurlCommand: control characters', () => {
     data: btoa('some=\u0007 \u0009 \u000C \u001B&other=param'),
   };
 
-  let command = convertRequestToCurlCommand(request);
+  const command = convertRequestToCurlCommand(request);
   expect(command).toEqual(
     "curl -v -X GET 'https://fbflipper.com/' -d $'some=\\u07 \\u09 \\u0c \\u1b&other=param'",
   );