Move checks inside

Author: gaearon

packages/react-reconciler/src/ReactFiberContext.js

@@ -134,12 +134,12 @@ function hasContextChanged(): boolean {
 }
 
 function isContextProvider(type: Function): boolean {
-  const childContextTypes = type.childContextTypes;
-  const hasChildContextTypes =
-    childContextTypes !== null && childContextTypes !== undefined;
   if (disableLegacyContext) {
     return false;
   } else {
+    const childContextTypes = type.childContextTypes;
+    const hasChildContextTypes =
+      childContextTypes !== null && childContextTypes !== undefined;
     return hasChildContextTypes;
   }
 }