feat: worker convenience methods (#2677)

This patch:
- adds `worker.evaluate` and `worker.evaluateHandle` methods as a shortcut to their execution context equivalents.
- makes the error messages a bit nicer when interacting with a closed worker (as opposed to a closed page).
- moves the worker tests into their own spec file.

test/worker.spec.js

@@ -0,0 +1,44 @@
+
+module.exports.addTests = function({testRunner, expect}) {
+  const {describe, xdescribe, fdescribe} = testRunner;
+  const {it, fit, xit} = testRunner;
+  const {beforeAll, beforeEach, afterAll, afterEach} = testRunner;
+
+  describe('Workers', function() {
+    it('Page.workers', async function({page, server}) {
+      await Promise.all([
+        new Promise(x => page.once('workercreated', x)),
+        page.goto(server.PREFIX + '/worker/worker.html')]);
+      const worker = page.workers()[0];
+      expect(worker.url()).toContain('worker.js');
+
+      expect(await worker.evaluate(() => self.workerFunction())).toBe('worker function result');
+
+      await page.goto(server.EMPTY_PAGE);
+      expect(page.workers()).toEqual([]);
+    });
+    it('should emit created and destroyed events', async function({page}) {
+      const workerCreatedPromise = new Promise(x => page.once('workercreated', x));
+      const workerObj = await page.evaluateHandle(() => new Worker('data:text/javascript,1'));
+      const worker = await workerCreatedPromise;
+      const workerThisObj = await worker.evaluateHandle(() => this);
+      const workerDestroyedPromise = new Promise(x => page.once('workerdestroyed', x));
+      await page.evaluate(workerObj => workerObj.terminate(), workerObj);
+      expect(await workerDestroyedPromise).toBe(worker);
+      const error = await workerThisObj.getProperty('self').catch(error => error);
+      expect(error.message).toContain('Most likely the worker has been closed.');
+    });
+    it('should report console logs', async function({page}) {
+      const logPromise = new Promise(x => page.on('console', x));
+      await page.evaluate(() => new Worker(`data:text/javascript,console.log(1)`));
+      const log = await logPromise;
+      expect(log.text()).toBe('1');
+    });
+    it('should have an execution context', async function({page}) {
+      const workerCreatedPromise = new Promise(x => page.once('workercreated', x));
+      await page.evaluate(() => new Worker(`data:text/javascript,console.log(1)`));
+      const worker = await workerCreatedPromise;
+      expect(await (await worker.executionContext()).evaluate('1+1')).toBe(2);
+    });
+  });
+};