chore: introduce //lib/api.js (#3835)

Introduce `//lib/api.js` that declares a list of publicly exposed
classes.

The `//lib/api.js` list superceedes dynamic `helper.tracePublicAPI()` calls
and is used in the following places:
- [ASYNC STACKS]: generate "async stacks" for publicy exposed API in `//index.js`
- [COVERAGE]: move coverage support from `//lib/helper` to `//test/utils`
- [DOCLINT]: get rid of 'exluded classes' hardcoded list

This will help us to re-use our coverage and doclint infrastructure
for Puppeteer-Firefox.

Drive-By: it turns out we didn't run coverage for `SecurityDetails`
class, so we lack coverage for a few methods there. These are excluded
for now, sanity tests will be added in a follow-up.

test/utils.js

@@ -18,7 +18,51 @@ const fs = require('fs');
 const path = require('path');
 const PROJECT_ROOT = fs.existsSync(path.join(__dirname, '..', 'package.json')) ? path.join(__dirname, '..') : path.join(__dirname, '..', '..');
 
+/**
+ * @param {Map<string, boolean>} apiCoverage
+ * @param {string} className
+ * @param {!Object} classType
+ */
+function traceAPICoverage(apiCoverage, className, classType) {
+  className = className.substring(0, 1).toLowerCase() + className.substring(1);
+  for (const methodName of Reflect.ownKeys(classType.prototype)) {
+    const method = Reflect.get(classType.prototype, methodName);
+    if (methodName === 'constructor' || typeof methodName !== 'string' || methodName.startsWith('_') || typeof method !== 'function')
+      continue;
+    apiCoverage.set(`${className}.${methodName}`, false);
+    Reflect.set(classType.prototype, methodName, function(...args) {
+      apiCoverage.set(`${className}.${methodName}`, true);
+      return method.call(this, ...args);
+    });
+  }
+
+  if (classType.Events) {
+    for (const event of Object.values(classType.Events))
+      apiCoverage.set(`${className}.emit(${JSON.stringify(event)})`, false);
+    const method = Reflect.get(classType.prototype, 'emit');
+    Reflect.set(classType.prototype, 'emit', function(event, ...args) {
+      if (this.listenerCount(event))
+        apiCoverage.set(`${className}.emit(${JSON.stringify(event)})`, true);
+      return method.call(this, event, ...args);
+    });
+  }
+}
+
 const utils = module.exports = {
+  recordAPICoverage: function(testRunner, api, disabled) {
+    const coverage = new Map();
+    for (const [className, classType] of Object.entries(api))
+      traceAPICoverage(coverage, className, classType);
+    testRunner.describe('COVERAGE', () => {
+      for (const method of coverage.keys()) {
+        (disabled.has(method) ? testRunner.xit : testRunner.it)(`public api '${method}' should be called`, async({page, server}) => {
+          if (!coverage.get(method))
+            throw new Error('NOT CALLED!');
+        });
+      }
+    });
+  },
+
   /**
    * @return {string}
    */