It seems like react-is v16.6 no longer is compatible with React v16.3 - v16.5, in that React v16.3 - v16.5 will use `Symbol(react.async_mode)`, but `react-is`'s `AsyncMode` export will instead be `Symbol(react.concurrent_mode)`.

It seems like the async mode symbol needs to remain the same in react-is 16.6+, and the `isAsyncMode` function would need to check *both* the AsyncMode and ConcurrentMode symbols.
* @emails react-core
 */
'use strict';
describe('ReactSymbols', () => {
  beforeEach(() => jest.resetModules());
  const expectToBeUnique = keyValuePairs => {
    const map = new Map();
    keyValuePairs.forEach(([key, value]) => {
      if (map.has(value)) {
        throw Error(
          `${key} value ${value.toString()} is the same as ${map.get(value)}.`,
        );
      }
      map.set(value, key);
    });
  };
  it('Symbol values should be unique', () => {
    expectToBeUnique(Object.entries(require('shared/ReactSymbols')));
  });
  it('numeric values should be unique', () => {
    const originalSymbolFor = global.Symbol.for;
    global.Symbol.for = null;
    try {
      expectToBeUnique(Object.entries(require('shared/ReactSymbols')));
    } finally {
      global.Symbol.for = originalSymbolFor;
    }
  });
});