jest: Fix jestjs/jest#10221 locally.

This is much easier than forking React Native, as we considered in
zulip/react-native#5. See
  jestjs/jest#10221 (comment).

jest.config.js

@@ -30,8 +30,25 @@ const projectForPlatform = platform => {
   }
   return {
     displayName: platform,
-    // See https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms.
-    preset: platform === 'ios' ? 'jest-expo/ios' : 'jest-expo/android',
+
+    // Ideally, these would simply be `jest-expo/ios` and
+    // `jest-expo/android`; see
+    // https://github.com/expo/expo/blob/master/packages/jest-expo/README.md#platforms.
+    // These custom presets are a workaround for a bug:
+    //
+    // `jest-expo`'s presets are based on `react-native`'s preset,
+    // which does something messy: it overwrites the global `Promise`.
+    // That's facebook/react-native#29303. Jest doesn't work well with
+    // that; that's facebook/jest#10221.
+    //
+    // So, until one of those is fixed, we use these custom presets to
+    // sandwich the code that replaces `global.Promise` with a fix:
+    //
+    // 1) save `global.Promise` to something else on `global`
+    // 2) let the `react-native` preset do its thing (like mocking RN
+    //    libraries)
+    // 3) assign `global.Promise` back to what we saved in step 1
+    preset: platform === 'ios' ? './jest/presetIos' : './jest/presetAndroid',
 
     // Finding and transforming source code.
     testPathIgnorePatterns: ['/node_modules/', '/src/__tests__/lib/', '-testlib.js$'],

jest/presetAndroid.js

@@ -0,0 +1,14 @@
+/**
+ * The preset we're making tweaks to.
+ */
+const basePreset = require('../node_modules/jest-expo/android/jest-preset.js');
+
+// See comment in our Jest config about how this is used.
+module.exports = {
+  ...basePreset,
+  setupFiles: [
+    require.resolve('./savePromise.js'),
+    ...basePreset.setupFiles,
+    require.resolve('./restorePromise.js'),
+  ],
+};

jest/presetIos.js

@@ -0,0 +1,14 @@
+/**
+ * The preset we're making tweaks to.
+ */
+const basePreset = require('../node_modules/jest-expo/ios/jest-preset.js');
+
+// See comment in our Jest config about how this is used.
+module.exports = {
+  ...basePreset,
+  setupFiles: [
+    require.resolve('./savePromise.js'),
+    ...basePreset.setupFiles,
+    require.resolve('./restorePromise.js'),
+  ],
+};

jest/restorePromise.js

@@ -0,0 +1,3 @@
+// For facebook/jest#10221. After savePromise.js and Jest's setup
+// files have run, restore the natural value of `global.Promise`.
+global.Promise = global.originalPromise;

jest/savePromise.js

@@ -0,0 +1,4 @@
+// For facebook/jest#10221. Before Jest's setup files have run, take
+// note of what the natural value of `global.Promise` is, so we can
+// restore it in restorePromise.js.
+global.originalPromise = Promise;