Prepend passed sourceExts to default ones and pass them to metro (#21855)

Summary:
Fixes react-native start not using passed --sourceExts #21854
Pull Request resolved: https://github.com/facebook/react-native/pull/21855

Differential Revision: D12840358

Pulled By: rafeca

fbshipit-source-id: 4ee09341b5128d83274a39d8d01c13749efaa78b

local-cli/server/runServer.js

@@ -70,6 +70,13 @@ async function runServer(args: Args, config: ConfigT) {
   config.server.enhanceMiddleware = middleware =>
     middlewareManager.getConnectInstance().use(middleware);
 
+  if (args.sourceExts !== config.resolver.sourceExts) {
+    // $FlowFixMe Metro configuration is immutable.
+    config.resolver.sourceExts = args.sourceExts.concat(
+      config.resolver.sourceExts,
+    );
+  }
+
   const serverInstance = await Metro.runServer(config, {
     host: args.host,
     secure: args.https,