Conditionalize the Regenerator Setup

Summary:
Changelog: [Internal]

If generators are provided natively, that should suggest that the JS source
code did not go through the regenerator-transform (e.g. in Metro Hermes profile),
then there is no need to set up the regenerator runtime.

This should save some work during the Core initialization.

Reviewed By: motiz88

Differential Revision: D29986751

fbshipit-source-id: 129f5122e8e4c05535ee2aa5da6970a66843e8cd

Libraries/Core/setUpRegeneratorRuntime.js

@@ -11,17 +11,34 @@
 'use strict';
 
 const {polyfillGlobal} = require('../Utilities/PolyfillFunctions');
+const {hasNativeConstructor} = require('../Utilities/FeatureDetection');
 
 /**
  * Set up regenerator.
  * You can use this module directly, or just require InitializeCore.
  */
-polyfillGlobal('regeneratorRuntime', () => {
-  // The require just sets up the global, so make sure when we first
-  // invoke it the global does not exist
-  delete global.regeneratorRuntime;
 
-  // regenerator-runtime/runtime exports the regeneratorRuntime object, so we
-  // can return it safely.
-  return require('regenerator-runtime/runtime'); // flowlint-line untyped-import:off
-});
+let hasNativeGenerator;
+try {
+  // If this function was lowered by regenerator-transform, it will try to
+  // access `global.regeneratorRuntime` which doesn't exist yet and will throw.
+  hasNativeGenerator = hasNativeConstructor(function*() {},
+  'GeneratorFunction');
+} catch {
+  // In this case, we know generators are not provided natively.
+  hasNativeGenerator = false;
+}
+
+// If generators are provided natively, which suggests that there was no
+// regenerator-transform, then there is no need to set up the runtime.
+if (!hasNativeGenerator) {
+  polyfillGlobal('regeneratorRuntime', () => {
+    // The require just sets up the global, so make sure when we first
+    // invoke it the global does not exist
+    delete global.regeneratorRuntime;
+
+    // regenerator-runtime/runtime exports the regeneratorRuntime object, so we
+    // can return it safely.
+    return require('regenerator-runtime/runtime'); // flowlint-line untyped-import:off
+  });
+}

Libraries/Utilities/FeatureDetection.js

@@ -19,4 +19,13 @@ function isNativeFunction(f: Function): boolean {
   return typeof f === 'function' && f.toString().indexOf('[native code]') > -1;
 }
 
-module.exports = {isNativeFunction};
+/**
+ * @return whether or not the constructor of @param {object} o is an native
+ * function named with @param {string} expectedName.
+ */
+function hasNativeConstructor(o: Object, expectedName: string): boolean {
+  const con = Object.getPrototypeOf(o).constructor;
+  return con.name === expectedName && isNativeFunction(con);
+}
+
+module.exports = {isNativeFunction, hasNativeConstructor};