Migrate WelcomeScreen

Summary: Migrated WelcomeScreen.js to WelcomeScreen.tsx

Reviewed By: passy

Differential Revision: D16667903

fbshipit-source-id: 86814d52175e611961733d608c34ef1cdbf8db1a

src/App.js

@@ -8,7 +8,7 @@
 import React from 'react';
 import {FlexColumn, FlexRow} from 'flipper';
 import {connect} from 'react-redux';
-import WelcomeScreen from './chrome/WelcomeScreen.js';
+import WelcomeScreen from './chrome/WelcomeScreen.tsx';
 // $FlowFixMe T47375728
 import TitleBar from './chrome/TitleBar.tsx';
 import MainSidebar from './chrome/MainSidebar.js';

src/chrome/WelcomeScreen.tsx

@@ -7,7 +7,6 @@
 
 import {
   styled,
-  PureComponent,
   FlexColumn,
   FlexRow,
   Text,
@@ -17,6 +16,7 @@ import {
 } from 'flipper';
 import isProduction from '../utils/isProduction.js';
 import {shell, remote} from 'electron';
+import React, {PureComponent} from 'react';
 
 const Container = styled(FlexColumn)({
   height: '100%',
@@ -98,7 +98,7 @@ export default class WelcomeScreen extends PureComponent<Props, State> {
     isMounted: false,
   };
 
-  timer: ?TimeoutID;
+  timer: NodeJS.Timeout | null | undefined;
 
   componentDidMount() {
     // waiting sometime before showing the welcome screen to allow Flipper to