[Suspense] Use !important to hide Suspended nodes (facebook#15861)

Suspended nodes are hidden using an inline `display: none` style. We do
this instead of removing the nodes from the DOM so that their state is
preserved when they are shown again.

Inline styles have the greatest specificity, but they are superseded by
`!important`. To prevent an external style from overriding React's, this
commit changes the hidden style to `display: none !important`.

MaYBE AnDREw sHOulD JusT LEArn Css

I attempted to write a unit test using `getComputedStyle` but JSDOM
doesn't respect `!important`. I think our existing tests are sufficient
but if we were to decide we need something more robust, I would set up
an e2e test.

packages/react-dom/src/__tests__/ReactDOMSuspensePlaceholder-test.js

@@ -90,9 +90,9 @@ describe('ReactDOMSuspensePlaceholder', () => {
       );
     }
     ReactDOM.render(<App />, container);
-    expect(divs[0].current.style.display).toEqual('none');
-    expect(divs[1].current.style.display).toEqual('none');
-    expect(divs[2].current.style.display).toEqual('none');
+    expect(divs[0].current.style.display).toEqual('none !important');
+    expect(divs[1].current.style.display).toEqual('none !important');
+    expect(divs[2].current.style.display).toEqual('none !important');
 
     await advanceTimers(500);
 
@@ -156,12 +156,14 @@ describe('ReactDOMSuspensePlaceholder', () => {
         ReactDOM.render(<App />, container);
       });
       expect(container.innerHTML).toEqual(
-        '<span style="display: none;">Sibling</span><span style="display: none;"></span>Loading...',
+        '<span style="display: none !important;">Sibling</span><span style=' +
+          '"display: none !important;"></span>Loading...',
       );
 
       act(() => setIsVisible(true));
       expect(container.innerHTML).toEqual(
-        '<span style="display: none;">Sibling</span><span style="display: none;"></span>Loading...',
+        '<span style="display: none !important;">Sibling</span><span style=' +
+          '"display: none !important;"></span>Loading...',
       );
 
       await advanceTimers(500);

packages/react-dom/src/client/ReactDOMHostConfig.js

@@ -589,7 +589,7 @@ export function hideInstance(instance: Instance): void {
   // TODO: Does this work for all element types? What about MathML? Should we
   // pass host context to this method?
   instance = ((instance: any): HTMLElement);
-  instance.style.display = 'none';
+  instance.style.display = 'none !important';
 }
 
 export function hideTextInstance(textInstance: TextInstance): void {

packages/react-fresh/src/__tests__/ReactFresh-test.js

@@ -1359,7 +1359,7 @@ describe('ReactFresh', () => {
       const fallbackChild = container.childNodes[1];
       expect(primaryChild.textContent).toBe('Content 1');
       expect(primaryChild.style.color).toBe('green');
-      expect(primaryChild.style.display).toBe('none');
+      expect(primaryChild.style.display).toBe('none !important');
       expect(fallbackChild.textContent).toBe('Fallback 0');
       expect(fallbackChild.style.color).toBe('green');
       expect(fallbackChild.style.display).toBe('');
@@ -1373,7 +1373,7 @@ describe('ReactFresh', () => {
       expect(container.childNodes[1]).toBe(fallbackChild);
       expect(primaryChild.textContent).toBe('Content 1');
       expect(primaryChild.style.color).toBe('green');
-      expect(primaryChild.style.display).toBe('none');
+      expect(primaryChild.style.display).toBe('none !important');
       expect(fallbackChild.textContent).toBe('Fallback 1');
       expect(fallbackChild.style.color).toBe('green');
       expect(fallbackChild.style.display).toBe('');
@@ -1397,7 +1397,7 @@ describe('ReactFresh', () => {
       expect(container.childNodes[1]).toBe(fallbackChild);
       expect(primaryChild.textContent).toBe('Content 1');
       expect(primaryChild.style.color).toBe('red');
-      expect(primaryChild.style.display).toBe('none');
+      expect(primaryChild.style.display).toBe('none !important');
       expect(fallbackChild.textContent).toBe('Fallback 1');
       expect(fallbackChild.style.color).toBe('red');
       expect(fallbackChild.style.display).toBe('');