[react-dom] expose TestUtils.act()

lib/react-dom.js

@@ -61,6 +61,10 @@ declare module 'react-dom/server' {
   declare var version: string;
 }
 
+type Thenable = {
+  then(resolve: () => mixed, reject?: () => mixed): mixed,
+};
+
 declare module 'react-dom/test-utils' {
   declare var Simulate: {
     [eventName: string]: (element: Element, eventData?: Object) => void,
@@ -113,6 +117,7 @@ declare module 'react-dom/test-utils' {
     tree: React$Component<any, any>,
     componentClass: React$ElementType,
   ): ?React$Component<any, any>;
+  declare function act<X>(callback: () => void): Thenable;
 }
 
 declare class SyntheticEvent<+T: EventTarget = EventTarget, +E: Event = Event> {

tests/react/react.diff

@@ -0,0 +1,43 @@
+--- react.exp
++++ react.out
+@@ -3093,11 +3093,29 @@
+                   ^^^^^^^^^^^^^
+
+ References:
+-   <BUILTINS>/react-dom.js:90:19
+-   90|     test: (child: React$Component<any, any>) => boolean,
++   <BUILTINS>/react-dom.js:94:19
++   94|     test: (child: React$Component<any, any>) => boolean,
+                          ^^^^^^^^^^^^^^^^^^^^^^^^^ [1]
+
+
++Error ---------------------------------------------------------------------------------------------- test-utils.js:57:15
++
++Cannot call `TestUtils.act` with function bound to `callback` because object literal [1] is incompatible with
++undefined [2] in the return value.
++
++   test-utils.js:57:15
++    57| TestUtils.act(() => ({count: 123})); // error
++                      ^^^^^^^^^^^^^^^^^^^^
++
++References:
++   test-utils.js:57:22
++    57| TestUtils.act(() => ({count: 123})); // error
++                             ^^^^^^^^^^^^ [1]
++   <BUILTINS>/react-dom.js:120:43
++   120|   declare function act<X>(callback: () => void): Thenable;
++                                                  ^^^^ [2]
++
++
+ Error -------------------------------------------------------------------------------------------- undefined_prop.js:7:1
+
+ Cannot create `MyComponent` element because undefined [1] is incompatible with string [2] in property `title`.
+@@ -4107,7 +4125,7 @@
+
+
+
+-Found 241 errors
++Found 242 errors
+
+ Only showing the most relevant union/intersection branches.
+ To see all branches, re-run Flow with --show-all-branches

tests/react/test-utils.js

@@ -51,3 +51,7 @@ if (buttonEl != null) {
   tree,
   MyTestingComponent,
 ): ?React.Component<any, any>);
+TestUtils.act(() => {
+  Math.random();
+});
+TestUtils.act(() => ({count: 123})); // error

tests/react_16_3/react_16_3.diff

@@ -0,0 +1,24 @@
+--- react_16_3.exp
++++ react_16_3.out
+@@ -40,8 +40,8 @@
+    forwardRef.js:19:32
+     19| const badRef = React.createRef<HTMLDivElement>();
+                                        ^^^^^^^^^^^^^^ [1]
+-   <BUILTINS>/react-dom.js:284:22
+-   284|   button: {instance: HTMLButtonElement, props: {children?: React$Node, [key: string]: any}},
++   <BUILTINS>/react-dom.js:289:22
++   289|   button: {instance: HTMLButtonElement, props: {children?: React$Node, [key: string]: any}},
+                              ^^^^^^^^^^^^^^^^^ [2]
+    <BUILTINS>/react.js:240:6
+    240|   ): {current: null | T};
+@@ -71,8 +71,8 @@
+    forwardRef.js:23:50
+     23| const _g =  <FancyButton foo={3} ref={(x: null | HTMLDivElement) => x} />; // Incorrect ref type
+                                                          ^^^^^^^^^^^^^^ [3]
+-   <BUILTINS>/react-dom.js:284:22
+-   284|   button: {instance: HTMLButtonElement, props: {children?: React$Node, [key: string]: any}},
++   <BUILTINS>/react-dom.js:289:22
++   289|   button: {instance: HTMLButtonElement, props: {children?: React$Node, [key: string]: any}},
+                              ^^^^^^^^^^^^^^^^^ [4]
+
+