Fix return type of StyleSheet.create to be the identity function

Summary:
Changes the return type of StyleSheet.create to `$ReadOnly<S>`. At runtime, it just freezes the fields in dev mode and is the identity function otherwise. This cause 423 errors due to the wrong props being passed to different styles: P124372727

I went over errors at random with Eli to get confidence in the change.

Changelog:
[General][Fixed] Fix return type of StyleSheet.create

Reviewed By: zackargyle, TheSavior

Differential Revision: D19218817

fbshipit-source-id: b7aa10700f8333372c9f0bd61bfbef371caadf97

Libraries/LogBox/UI/LogBoxNotification.js

@@ -60,6 +60,8 @@ function LogBoxLogNotification(props: Props): React.Node {
 function CountBadge(props) {
   return (
     <View style={countStyles.outside}>
+      {/* $FlowFixMe(>=0.114.0) This suppression was added when fixing the type
+       * of `StyleSheet.create`. Remove this comment to see the error. */}
       <View style={[countStyles.inside, countStyles[props.level]]}>
         <Text style={countStyles.text}>
           {props.count <= 1 ? '!' : props.count}

Libraries/StyleSheet/StyleSheet.js

@@ -347,7 +347,7 @@ module.exports = {
   /**
    * Creates a StyleSheet style reference from the given object.
    */
-  create<+S: ____Styles_Internal>(obj: S): $ObjMap<S, (Object) => any> {
+  create<+S: ____Styles_Internal>(obj: S): $ReadOnly<S> {
     // TODO: This should return S as the return type. But first,
     // we need to codemod all the callsites that are typing this
     // return value as a number (even though it was opaque).