fix(downloads): fix acceptDownloads complaint (microsoft#1777)

This patch fixes `download.path()` throwing an error that
`acceptDownloads` is not set when using a download within a `page.on`
handler even when `acceptDownloads` had been set to `true`;

Fixes microsoft#1777

src/download.ts

@@ -41,9 +41,9 @@ export class Download {
     this._finishedPromise = new Promise(f => this._finishedCallback = f);
     for (const barrier of this._page._frameManager._signalBarriers)
       barrier.addDownload();
-    this._page.emit(Events.Page.Download, this);
     page._browserContext._downloads.add(this);
     this._acceptDownloads = !!this._page._browserContext._options.acceptDownloads;
+    this._page.emit(Events.Page.Download, this);
   }
 
   url(): string {

test/download.spec.js

@@ -70,7 +70,7 @@ describe('Download', function() {
     expect(fs.readFileSync(path).toString()).toBe('Hello world');
     await page.close();
   });
-  fit(`should report download path within page.on('download', …) handler`, async({browser, server}) => {
+  it(`should report download path within page.on('download', …) handler`, async({browser, server}) => {
     const page = await browser.newPage({ acceptDownloads: true });
     const onDownloadPathPath = new Promise((res, rej) => {
       setTimeout(() => { rej('failed to find path quickly'); }, 1000);
@@ -83,6 +83,7 @@ describe('Download', function() {
     await page.click('a');
     const path = await onDownloadPathPath;
     expect(fs.readFileSync(path).toString()).toBe('Hello world');
+    await page.close();
   })
   it.skip(FFOX).fail(CHROMIUM || WEBKIT)('should report alt-click downloads', async({browser, server}) => {
     // Firefox does not download on alt-click by default.