Implement public API coverage

This patch:
- implements a basic public API coverage based on 'helper.tracePublicAPI' methods
- adds `npm run coverage` command which reports coverage after running all of the unit tests

References #50.

lib/helper.js

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+/** @type {?Map<string, boolean>} */
+let apiCoverage = null;
 class Helper {
   /**
    * @param {function()|string} fun
@@ -116,16 +118,21 @@ class Helper {
     let className = classType.prototype.constructor.name;
     className = className.substring(0, 1).toLowerCase() + className.substring(1);
     const debug = require('debug')(`puppeteer:${className}`);
-    if (!debug.enabled)
+    if (!debug.enabled && !apiCoverage)
       return;
     for (let methodName of Reflect.ownKeys(classType.prototype)) {
       const method = Reflect.get(classType.prototype, methodName);
       if (methodName === 'constructor' || typeof methodName !== 'string' || methodName.startsWith('_') || typeof method !== 'function')
         continue;
+      if (apiCoverage)
+        apiCoverage.set(`${className}.${methodName}`, false);
       Reflect.set(classType.prototype, methodName, function(...args) {
         let argsText = args.map(stringifyArgument).join(', ');
         let callsite = `${className}.${methodName}(${argsText})`;
-        debug(callsite);
+        if (debug.enabled)
+          debug(callsite);
+        if (apiCoverage)
+          apiCoverage.set(`${className}.${methodName}`, true);
         return method.call(this, ...args);
       });
     }
@@ -163,6 +170,17 @@ class Helper {
       listener.emitter.removeListener(listener.eventName, listener.handler);
     listeners.splice(0, listeners.length);
   }
+
+  /**
+   * @return {?Map<string, boolean>}
+   */
+  static publicAPICoverage() {
+    return apiCoverage;
+  }
+
+  static recordPublicAPICoverage() {
+    apiCoverage = new Map();
+  }
 }
 
 module.exports = Helper;

package.json

@@ -11,10 +11,11 @@
     "debug-unit": "DEBUG_TEST=true node --inspect-brk ./node_modules/.bin/jasmine test/test.js",
     "test-phantom": "python third_party/phantomjs/test/run-tests.py",
     "test-doclint": "jasmine utils/doclint/test/test.js",
-    "test": "npm run lint --silent && npm run unit && npm run test-phantom && npm run test-doclint",
+    "test": "npm run lint --silent && npm run coverage && npm run test-phantom && npm run test-doclint",
     "install": "node install.js",
     "lint": "([ \"$CI\" = true ] && eslint --quiet -f codeframe . || eslint .) && npm run doc",
-    "doc": "node utils/doclint/cli.js"
+    "doc": "node utils/doclint/cli.js",
+    "coverage": "COVERAGE=true npm run unit"
   },
   "author": "The Chromium Authors",
   "license": "SEE LICENSE IN LICENSE",

test/test.js

@@ -17,6 +17,9 @@
 let fs = require('fs');
 let rm = require('rimraf').sync;
 let path = require('path');
+let helper = require('../lib/helper');
+if (process.env.COVERAGE)
+  helper.recordPublicAPICoverage();
 let Browser = require('../lib/Browser');
 let SimpleServer = require('./server/SimpleServer');
 let GoldenUtils = require('./golden-utils');
@@ -1345,6 +1348,24 @@ describe('Puppeteer', function() {
   });
 });
 
+if (process.env.COVERAGE) {
+  describe('API', function(){
+    let coverage = helper.publicAPICoverage();
+    let disabled = new Set();
+    if (headless) {
+      disabled.add('dialog.accept');
+      disabled.add('dialog.dismiss');
+    } else {
+      disabled.add('page.pdf');
+    }
+
+    for (let method of coverage.keys()) {
+      (disabled.has(method) ? xit : it)(`public method '${method}' was tested`, SX(async function(){
+        expect(publicAPICoverage.get(method)).toBe(true);
+      }));
+    }
+  });
+}
 /**
  * @param {!EventEmitter} emitter
  * @param {string} eventName