fix babel 'strict mode' error

Summary:
need to remove function declaration in a lexically nested statement
because babel uses 'use strict' by default now
Closes https://github.com/facebook/react-native/pull/11390

Differential Revision: D4308275

fbshipit-source-id: 0d073361d25b23fb67f001225feb72532af38683

Libraries/Core/InitializeCore.js

@@ -109,7 +109,7 @@ require('RCTLog');
 
 // Set up error handler
 if (!global.__fbDisableExceptionsManager) {
-  function handleError(e, isFatal) {
+  const handleError = (e, isFatal) => {
     try {
       ExceptionsManager.handleException(e, isFatal);
     } catch (ee) {
@@ -118,7 +118,7 @@ if (!global.__fbDisableExceptionsManager) {
       /* eslint-enable no-console-disallow */
       throw e;
     }
-  }
+  };
 
   const ErrorUtils = require('ErrorUtils');
   ErrorUtils.setGlobalHandler(handleError);