[fix] StyleSheet SSR API

Without a getter, an outdated version of `sheet` is returned after
`getStyleSheet` is called. This is exposed in the public API via
`AppRegistry.getApplication`.

Close #1874

packages/react-native-web/src/exports/StyleSheet/__tests__/createStyleResolver-test.js

@@ -1,9 +1,11 @@
 /* eslint-env jasmine, jest */
 
+import ExecutionEnvironment from 'fbjs/lib/ExecutionEnvironment';
 import I18nManager from '../../I18nManager';
 import ReactNativePropRegistry from '../ReactNativePropRegistry';
 import createStyleResolver from '../createStyleResolver';
 
+const canUseDOM = ExecutionEnvironment.canUseDOM;
 let styleResolver;
 
 describe('StyleSheet/createStyleResolver', () => {
@@ -74,5 +76,24 @@ describe('StyleSheet/createStyleResolver', () => {
     test('resolves inline-style pointerEvents to classname', () => {
       expect(styleResolver.resolve({ pointerEvents: 'box-none' })).toMatchSnapshot();
     });
+
+    describe('sheet', () => {
+      beforeEach(() => {
+        ExecutionEnvironment.canUseDOM = false;
+      });
+
+      afterEach(() => {
+        ExecutionEnvironment.canUseDOM = canUseDOM;
+      });
+
+      test('returns the new sheet once re-initialized', () => {
+        const sheet = styleResolver.sheet;
+
+        // re-initialize the sheet
+        styleResolver.getStyleSheet();
+
+        expect(styleResolver.sheet).not.toBe(sheet);
+      });
+    });
   });
 });

packages/react-native-web/src/exports/StyleSheet/createStyleResolver.js

@@ -237,7 +237,9 @@ export default function createStyleResolver() {
       return result;
     },
     resolve,
-    sheet
+    get sheet() {
+      return sheet;
+    }
   };
 }