Add keyboardAppearance prop to TextInput component.

Summary: Resolves #3649.
Closes https://github.com/facebook/react-native/pull/4012

Reviewed By: javache

Differential Revision: D2636538

Pulled By: nicklockwood

fb-gh-sync-id: 022e79d8f8fa684cad43af1a51f728d60ac652a8

Examples/UIExplorer/TextInputExample.ios.js

@@ -386,6 +386,27 @@ exports.examples = [
       return <View>{examples}</View>;
     }
   },
+  {
+    title: 'Keyboard appearance',
+    render: function() {
+      var keyboardAppearance = [
+        'default',
+        'light',
+        'dark',
+      ];
+      var examples = keyboardAppearance.map((type) => {
+        return (
+          <WithLabel key={type} label={type}>
+            <TextInput
+              keyboardAppearance={type}
+              style={styles.default}
+            />
+          </WithLabel>
+        );
+      });
+      return <View>{examples}</View>;
+    }
+  },
   {
     title: 'Return key types',
     render: function() {