Add Flow libdefs for HermesInternalType

Summary:
Changelog: [Internal]

This diff add a flow libdefs for the `HermesInternalType` to type
`HermesInternal` as the first accurately typed `global` property,
and filled all the type holes.

Reviewed By: yungsters

Differential Revision: D29986749

fbshipit-source-id: a94be7919f989b5085f6b264e55145a85020fea9

Libraries/Core/polyfillPromise.js

@@ -29,7 +29,7 @@ if (global?.HermesInternal?.hasPromise?.()) {
     if (typeof HermesPromise !== 'function') {
       console.error('HermesPromise does not exist');
     }
-    global.HermesInternal.enablePromiseRejectionTracker(
+    global.HermesInternal?.enablePromiseRejectionTracker?.(
       require('../promiseRejectionTrackingOptions').default,
     );
   }

Libraries/Core/setUpTimers.js

@@ -21,8 +21,8 @@ if (__DEV__) {
 
 // Currently, Hermes `Promise` is implemented via Internal Bytecode.
 const hasHermesPromiseQueuedToJSVM =
-  global?.HermesInternal?.hasPromise?.() &&
-  global?.HermesInternal?.useEngineQueue?.();
+  global.HermesInternal?.hasPromise?.() === true &&
+  global.HermesInternal?.useEngineQueue?.() === true;
 
 const hasNativePromise = isNativeFunction(Promise);
 const hasPromiseQueuedToJSVM = hasNativePromise || hasHermesPromiseQueuedToJSVM;
@@ -83,7 +83,7 @@ if (hasPromiseQueuedToJSVM) {
  */
 if (hasHermesPromiseQueuedToJSVM) {
   // Fast path for Hermes.
-  polyfillGlobal('queueMicrotask', () => global.HermesInternal.enqueueJob);
+  polyfillGlobal('queueMicrotask', () => global.HermesInternal?.enqueueJob);
 } else {
   // Polyfill it with promise (regardless it's polyfiled or native) otherwise.
   polyfillGlobal(

flow/HermesInternalType.js

@@ -0,0 +1,114 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict
+ * @format
+ */
+
+// Declarations for functionality exposed by the Hermes VM.
+//
+// For backwards-compatibility, code that uses such functionality must also
+// check explicitly at run-time whether the object(s) and method(s) exist, and
+// fail safely if not.
+
+/**
+ * HermesInternalType is an object containing functions used to interact with
+ * the VM in a way that is not standardized by the JS spec.
+ * There are limited guarantees about these functions, and they should not be
+ * widely used. Consult with the Hermes team before using any of these.
+ * There may be other visible properties on this object; however, those are
+ * only exposed for testing purposes: do not use them.
+ */
+declare type $HermesInternalType = {
+  // All members are optional because they may not exist when OTA'd to older
+  // VMs.
+
+  +getNumGCs?: () => number,
+  +getGCTime?: () => number,
+  +getNativeCallTime?: () => number,
+  +getNativeCallCount?: () => number,
+  +getGCCPUTime?: () => number,
+
+  /**
+   * Hermes can embed an "epilogue" to the bytecode file with arbitrary bytes.
+   * At most one epilogue will exist per bytecode module (which can be
+   * different than a JS module).
+   * Calling this function will return all such epilogues and convert the
+   * bytes to numbers in the range of 0-255.
+   */
+  +getEpilogues?: () => Array<Array<number>>,
+
+  /**
+   * Query the VM for various statistics about performance.
+   * There are no guarantees about what keys exist in it, but they can be
+   * printed for informational purposes.
+   * @return An object that maps strings to various types of performance
+   *    statistics.
+   */
+  +getInstrumentedStats?: () => {[string]: number | string, ...},
+
+  /**
+   * Query the VM for any sort of runtime properties that it wants to report.
+   * There are no guarantees about what keys exist in it, but they can be
+   * printed for informational purposes.
+   * @return An object that maps strings to various types of runtime properties.
+   */
+  +getRuntimeProperties?: () => {
+    'OSS Release Version': string,
+    'Build': string,
+    [string]: mixed,
+  },
+
+  /**
+   * Tell Hermes that at this point the surface has transitioned from TTI to
+   * post-TTI. The VM can change some of its internal behavior to optimize for
+   * post-TTI scenarios.
+   * This can be called several times but will have no effect after the first
+   * call.
+   */
+  +ttiReached?: () => void,
+
+  /**
+   * Tell Hermes that at this point the surface has transitioned from TTRC to
+   * post-TTRC. The VM can change some of its internal behavior to optimize for
+   * post-TTRC scenarios.
+   * This can be called several times but will have no effect after the first
+   * call.
+   */
+  +ttrcReached?: () => void,
+
+  /**
+   * Query the VM to see whether or not it enabled Promise.
+   */
+  +hasPromise?: () => boolean,
+
+  /**
+   * Enable promise rejection tracking with the given options.
+   * The API mirrored the `promise` npm package, therefore it's typed same as
+   * the `enable` function of module `promise/setimmediate/rejection-tracking`
+   * declared in ./flow-typed/npm/promise_v8.x.x.js.
+   */
+  +enablePromiseRejectionTracker?: (
+    options: ?{
+      whitelist?: ?Array<mixed>,
+      allRejections?: ?boolean,
+      onUnhandled?: ?(number, mixed) => void,
+      onHandled?: ?(number, mixed) => void,
+    },
+  ) => void,
+
+  /**
+   * Query the VM to see whether or not it use the engine Job queue.
+   */
+  +useEngineQueue?: () => boolean,
+
+  /**
+   * Enqueue a JavaScript callback function as a Job into the engine Job queue.
+   */
+  +enqueueJob?: <TArguments: Array<mixed>>(
+    jobCallback: (...args: TArguments) => mixed,
+  ) => void,
+};

flow/global.js

@@ -16,6 +16,8 @@
  * writeability (`+`) when defining types.
  */
 declare var global: {
+  +HermesInternal: ?$HermesInternalType,
+
   // Undeclared properties are implicitly `any`.
   [string | symbol]: any,
 };