Disable legacy mode in Modern www builds

These already don't expose the entry points to create legacy roots.

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

@@ -14,7 +14,7 @@ const ReactDOM = require('react-dom');
 const StrictMode = React.StrictMode;
 
 describe('findDOMNode', () => {
-  // @gate www
+  // @gate www && !disableLegacyMode
   it('findDOMNode should return null if passed null', () => {
     expect(ReactDOM.findDOMNode(null)).toBe(null);
   });
@@ -66,7 +66,7 @@ describe('findDOMNode', () => {
     expect(b.tagName).toBe('SPAN');
   });
 
-  // @gate www
+  // @gate www && !disableLegacyMode
   it('findDOMNode should reject random objects', () => {
     expect(function () {
       ReactDOM.findDOMNode({foo: 'bar'});

packages/react-reconciler/src/__tests__/ReactScope-test.internal.js

@@ -393,7 +393,7 @@ describe('ReactScope', () => {
     });
 
     // @gate www
-    it('DO_NOT_USE_queryAllNodes() works as intended', () => {
+    it('DO_NOT_USE_queryAllNodes() works as intended', async () => {
       const testScopeQuery = (type, props) => true;
       const TestScope = React.unstable_Scope;
       const scopeRef = React.createRef();
@@ -417,20 +417,25 @@ describe('ReactScope', () => {
         );
       }
 
-      const renderer = ReactTestRenderer.create(<Test toggle={true} />, {
-        createNodeMock: element => {
-          return element;
-        },
-      });
+      let renderer;
+      await act(
+        () =>
+          (renderer = ReactTestRenderer.create(<Test toggle={true} />, {
+            createNodeMock: element => {
+              return element;
+            },
+            unstable_isConcurrent: true,
+          })),
+      );
       let nodes = scopeRef.current.DO_NOT_USE_queryAllNodes(testScopeQuery);
       expect(nodes).toEqual([divRef.current, spanRef.current, aRef.current]);
-      renderer.update(<Test toggle={false} />);
+      await act(() => renderer.update(<Test toggle={false} />));
       nodes = scopeRef.current.DO_NOT_USE_queryAllNodes(testScopeQuery);
       expect(nodes).toEqual([aRef.current, divRef.current, spanRef.current]);
     });
 
     // @gate www
-    it('DO_NOT_USE_queryFirstNode() works as intended', () => {
+    it('DO_NOT_USE_queryFirstNode() works as intended', async () => {
       const testScopeQuery = (type, props) => true;
       const TestScope = React.unstable_Scope;
       const scopeRef = React.createRef();
@@ -454,20 +459,26 @@ describe('ReactScope', () => {
         );
       }
 
-      const renderer = ReactTestRenderer.create(<Test toggle={true} />, {
-        createNodeMock: element => {
-          return element;
-        },
-      });
+      let renderer;
+      await act(
+        () =>
+          (renderer = ReactTestRenderer.create(<Test toggle={true} />, {
+            createNodeMock: element => {
+              return element;
+            },
+            unstable_isConcurrent: true,
+          })),
+      );
       let node = scopeRef.current.DO_NOT_USE_queryFirstNode(testScopeQuery);
       expect(node).toEqual(divRef.current);
-      renderer.update(<Test toggle={false} />);
+      await act(() => renderer.update(<Test toggle={false} />));
+
       node = scopeRef.current.DO_NOT_USE_queryFirstNode(testScopeQuery);
       expect(node).toEqual(aRef.current);
     });
 
     // @gate www
-    it('containsNode() works as intended', () => {
+    it('containsNode() works as intended', async () => {
       const TestScope = React.unstable_Scope;
       const scopeRef = React.createRef();
       const divRef = React.createRef();
@@ -500,23 +511,28 @@ describe('ReactScope', () => {
         );
       }
 
-      const renderer = ReactTestRenderer.create(<Test toggle={true} />, {
-        createNodeMock: element => {
-          return element;
-        },
-      });
+      let renderer;
+      await act(
+        () =>
+          (renderer = ReactTestRenderer.create(<Test toggle={true} />, {
+            createNodeMock: element => {
+              return element;
+            },
+            unstable_isConcurrent: true,
+          })),
+      );
       expect(scopeRef.current.containsNode(divRef.current)).toBe(true);
       expect(scopeRef.current.containsNode(spanRef.current)).toBe(true);
       expect(scopeRef.current.containsNode(aRef.current)).toBe(true);
       expect(scopeRef.current.containsNode(outerSpan.current)).toBe(false);
       expect(scopeRef.current.containsNode(emRef.current)).toBe(false);
-      renderer.update(<Test toggle={false} />);
+      await act(() => renderer.update(<Test toggle={false} />));
       expect(scopeRef.current.containsNode(divRef.current)).toBe(true);
       expect(scopeRef.current.containsNode(spanRef.current)).toBe(true);
       expect(scopeRef.current.containsNode(aRef.current)).toBe(true);
       expect(scopeRef.current.containsNode(outerSpan.current)).toBe(false);
       expect(scopeRef.current.containsNode(emRef.current)).toBe(true);
-      renderer.update(<Test toggle={true} />);
+      await act(() => renderer.update(<Test toggle={true} />));
       expect(scopeRef.current.containsNode(emRef.current)).toBe(false);
     });
   });

packages/react-reconciler/src/__tests__/ReactSuspense-test.internal.js

@@ -378,7 +378,7 @@ describe('ReactSuspense', () => {
     expect(container.textContent).toEqual('AB');
   });
 
-  // @gate forceConcurrentByDefaultForTesting
+  // @gate !disableLegacyMode && forceConcurrentByDefaultForTesting
   it(
     'interrupts current render when something suspends with a ' +
       "delay and we've already skipped over a lower priority update in " +

packages/shared/forks/ReactFeatureFlags.www.js

@@ -113,7 +113,7 @@ export const useModernStrictMode = true;
 // because JSX is an extremely hot path.
 export const disableStringRefs = false;
 
-export const disableLegacyMode = false;
+export const disableLegacyMode = __EXPERIMENTAL__;
 
 export const disableDOMTestUtils = false;