Define regeneratorRuntime globally as a side-effect, again.

In #353, I simplified the implementation of regenerator-runtime by
converting runtime.js to a normal CommonJS module. No more global object
detection, and no more sneaky capture/deletion of regeneratorRuntime.

Unfortunately, folks who use only @babel/polyfill and not
@babel/transform-runtime may have existing code that relies on a global
regeneratorRuntime variable. That code will eventually disappear
(:crossed-fingers:), but in the meantime we can continue supporting it by
simply ensuring that regeneratorRuntime is defined globally as a
side-effect of importing regenerator-runtime.

This turns out to be much easier than detecting the global object, as long
as runtime.js runs as a non-strict CommonJS module. See my code comments
for another fallback solution that should cover all remaining cases.

Background discussion:
babel/babel#7646 (comment)

Fixes #363.
Fixes #337.
Fixes #367.

packages/regenerator-runtime/package.json

@@ -10,6 +10,7 @@
     "generator",
     "async"
   ],
+  "sideEffects": true,
   "repository": {
     "type": "git",
     "url": "https://github.com/facebook/regenerator/tree/master/packages/regenerator-runtime"

packages/regenerator-runtime/runtime.js

@@ -5,7 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-var regeneratorRuntime = (function (exports) {
+var runtime = (function (exports) {
   "use strict";
 
   var Op = Object.prototype;
@@ -709,3 +709,18 @@ var regeneratorRuntime = (function (exports) {
   // the regeneratorRuntime variable at the top of this file.
   typeof module === "object" ? module.exports : {}
 ));
+
+try {
+  regeneratorRuntime = runtime;
+} catch (accidentalStrictMode) {
+  // This module should not be running in strict mode, so the above
+  // assignment should always work unless something is misconfigured. Just
+  // in case runtime.js accidentally runs in strict mode, we can escape
+  // strict mode using a global Function call. This could conceivably fail
+  // if a Content Security Policy forbids using Function, but in that case
+  // the proper solution is to fix the accidental strict mode problem. If
+  // you've misconfigured your bundler to force strict mode and applied a
+  // CSP to forbid Function, and you're not willing to fix either of those
+  // problems, please detail your unique predicament in a GitHub issue.
+  Function("r", "regeneratorRuntime = r")(runtime);
+}

test/runtime.js

@@ -5,4 +5,4 @@
  * LICENSE file in the root directory of this source tree.
  */
 
-global.regeneratorRuntime = require("regenerator-runtime");
+require("regenerator-runtime");