Event coverage and debugging (#160)

This patch introduces event coverage for DEBUG module and 
API coverage.

Closes #50.

lib/helper.js

@@ -137,13 +137,35 @@ class Helper {
       });
     }
 
+    if (classType.Events) {
+      if (apiCoverage) {
+        for (let 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) {
+        let argsText = [JSON.stringify(event)].concat(args.map(stringifyArgument)).join(', ');
+        if (debug.enabled)
+          debug(`${className}.emit(${argsText})`);
+        if (apiCoverage && this.listenerCount(event))
+          apiCoverage.set(`${className}.emit(${JSON.stringify(event)})`, true);
+        return method.call(this, event, ...args);
+      });
+    }
+
     /**
      * @param {!Object} arg
      * @return {string}
      */
     function stringifyArgument(arg) {
-      if (typeof arg !== 'function')
+      if (typeof arg !== 'function') {
+        try {
           return JSON.stringify(arg);
+        } catch (e) {
+          // The object was recursive
+          return arg.toString();
+        }
+      }
       let text = arg.toString().split('\n').map(line => line.trim()).join('');
       if (text.length > 20)
         text = text.substring(0, 20) + '…';

test/test.js

@@ -1423,7 +1423,7 @@ if (process.env.COVERAGE) {
     }
 
     for (let method of coverage.keys()) {
-      (disabled.has(method) ? xit : it)(`public method '${method}' should be called`, SX(async function(){
+      (disabled.has(method) ? xit : it)(`public api '${method}' should be called`, SX(async function(){
         expect(coverage.get(method)).toBe(true);
       }));
     }