feat: introduce puppeteer-firefox (#3628)

This adds a proof-of-concept of `puppeteer-firefox`.
This consists of two parts:
- `//experimental/juggler` - patches to apply to Firefox.
- `//experimental/puppeteer-firefox` - front-end code to
be merged with Puppeteer.

As things become more stable, we'll gradually move it out of
the experimental folder.

experimental/puppeteer-firefox/test/dialog.spec.js

@@ -0,0 +1,39 @@
+const utils = require('./utils');
+
+module.exports.addTests = function({testRunner, expect, product}) {
+  const {describe, xdescribe, fdescribe} = testRunner;
+  const {it, fit, xit} = testRunner;
+  const {beforeAll, beforeEach, afterAll, afterEach} = testRunner;
+
+  const FFOX = product === 'firefox';
+  const CHROME = product === 'chromium';
+
+  describe('Page.Events.Dialog', function() {
+    it('should fire', async({page, server}) => {
+      page.on('dialog', dialog => {
+        expect(dialog.type()).toBe('alert');
+        expect(dialog.defaultValue()).toBe('');
+        expect(dialog.message()).toBe('yo');
+        dialog.accept();
+      });
+      await page.evaluate(() => alert('yo'));
+    });
+    it('should allow accepting prompts', async({page, server}) => {
+      page.on('dialog', dialog => {
+        expect(dialog.type()).toBe('prompt');
+        expect(dialog.defaultValue()).toBe('yes.');
+        expect(dialog.message()).toBe('question?');
+        dialog.accept('answer!');
+      });
+      const result = await page.evaluate(() => prompt('question?', 'yes.'));
+      expect(result).toBe('answer!');
+    });
+    it('should dismiss the prompt', async({page, server}) => {
+      page.on('dialog', dialog => {
+        dialog.dismiss();
+      });
+      const result = await page.evaluate(() => prompt('question?'));
+      expect(result).toBe(null);
+    });
+  });
+};