Fix TypeScript decorator support (#5783)

* Fix TypeScript decorator support

* Update babel flow override

* WIP

test/fixtures/typescript/src/App.test.ts

@@ -6,3 +6,9 @@ it('reads a typescript file with no syntax error', () => {
   expect(App.foo.baz!.n).toBe(123);
   expect(app.n).toBe(123);
 });
+
+it('supports decorators', () => {
+  const app = new App();
+  expect((app as any).annotated).toBe(true);
+  expect(app.decorated).toBe(42);
+});

test/fixtures/typescript/src/App.ts

@@ -10,10 +10,23 @@ type MyObject = Pick<MyType, 'bar' | 'baz'>;
 class App {
   static foo: MyObject = { bar: true, baz: { n: 123 } };
   n = App.foo.baz!.n;
+  @propertyDecorator
+  decorated;
 }
 
 function annotation(target: any) {
   target.annotated = true;
 }
 
+function propertyDecorator(target: any, key: string) {
+  if (delete target[key]) {
+    Object.defineProperty(target, key, {
+      get() {
+        return 42;
+      },
+      enumerable: true,
+    });
+  }
+}
+
 export default App;