[fix] onLayout event object

Make sure that `this` within the `target` getter references the React
component rather than the layout event object.

Fix #996

packages/react-native-web/src/exports/TextInput/index.js

@@ -183,6 +183,7 @@ class TextInput extends Component<*> {
       blurOnSubmit,
       clearTextOnFocus,
       onChangeText,
+      onLayout,
       onSelectionChange,
       onSubmitEditing,
       selection,

packages/react-native-web/src/modules/applyLayout/index.js

@@ -131,11 +131,12 @@ const applyLayout = Component => {
           ) {
             this._layoutState = { x, y, width, height };
             const nativeEvent = {
-              layout: this._layoutState,
-              get target() {
-                return findNodeHandle(this);
-              }
+              layout: this._layoutState
             };
+            Object.defineProperty(nativeEvent, 'target', {
+              enumerable: true,
+              get: () => findNodeHandle(this)
+            });
             onLayout({ nativeEvent, timeStamp: Date.now() });
           }
         }