🏗 Add support for gulp serve --coverage coverage collection endpoint (

ampproject#30099)

* add istanbul-middleware dependency

* Add coverage collection endpoint to gulp serve

* Switch middleware to devDependency

build-system/server/app.js

@@ -65,6 +65,9 @@ app.use('/amp4test', require('./amp4test').app);
 app.use('/analytics', require('./routes/analytics'));
 app.use('/list/', require('./routes/list'));
 app.use('/test', require('./routes/test'));
+if (argv.coverage) {
+  app.use('/coverage', require('istanbul-middleware').createHandler());
+}
 
 // Append ?csp=1 to the URL to turn on the CSP header.
 // TODO: shall we turn on CSP all the time?

build-system/tasks/serve.js

@@ -208,4 +208,5 @@ serve.flags = {
   esm: '  Serve ESM JS (requires the use of --new_server)',
   cdn: '  Serve current prod JS',
   rtv: '  Serve JS from the RTV provided',
+  coverage: '  Serve instrumented code to collect coverage info',
 };

package.json

@@ -121,6 +121,7 @@
     "gulp-watch": "5.0.1",
     "gzip-size": "5.1.1",
     "html-minifier": "4.0.0",
+    "istanbul-middleware": "0.2.2",
     "jest-dot-reporter": "1.0.12",
     "jest-silent-reporter": "0.2.1",
     "jison": "0.4.18",