Change the js signature to be Promise based and dynamically detect it used as a callback

Summary:
Making the default people see when they look at the module the Promise based version

public

Reviewed By: davidaurelio

Differential Revision: D2850048

fb-gh-sync-id: e0815983ed798c202047cb071e65ce63a52fd1af

Examples/UIExplorer/ClipboardExample.js

@@ -28,6 +28,7 @@ var ClipboardExample = React.createClass({
       content: 'Content will appear here'
     };
   },
+
   async _setClipboardContent(){
     Clipboard.setString('Hello World');
     try {
@@ -36,8 +37,8 @@ var ClipboardExample = React.createClass({
     } catch (e) {
       this.setState({content:e.message});
     }
-
   },
+
   render() {
     return (
       <View>

Libraries/Components/Clipboard/Clipboard.js

@@ -23,10 +23,10 @@ module.exports = {
    *   var content = await Clipboard.getString();
    * }
    * ```
-   * @param this parameter is deprecated. callback is function with one argument of string type
    */
-  getString(callback) {
-    if (callback) {
+  getString() {
+    if (arguments.length > 0) {
+        let callback = arguments[0];
         console.warn('Clipboard.getString(callback) is deprecated. Use the returned Promise instead');
         Clipboard.getString().then(callback);
         return;