pkg/docgen: refactoring and cleanup

pkg/docgen/lib/src/mdn.dart

+library docgen.mdn;
+
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:logging/logging.dart';
+import 'package:path/path.dart' as p;
+
+// TODO(janicejl): Make MDN content generic or pluggable.
+
+/// Map of all the comments for dom elements from MDN.
+Map<String, dynamic> _mdn;
+
+/// Generates MDN comments from database.json.
+String mdnComment(String root, Logger logger, String domName) {
+   //Check if MDN is loaded.
+ if (_mdn == null) {
+   // Reading in MDN related json file.
+   var mdnPath = p.join(root, 'utils/apidoc/mdn/database.json');
+   var mdnFile = new File(mdnPath);
+   if (mdnFile.existsSync()) {
+     _mdn = JSON.decode(mdnFile.readAsStringSync());
+   } else {
+     logger.warning("Cannot find MDN docs expected at $mdnPath");
+     _mdn = {};
+   }
+ }
+
+ var parts = domName.split('.');
+ if (parts.length == 2) return _mdnMemberComment(parts[0], parts[1]);
+ if (parts.length == 1) return _mdnTypeComment(parts[0]);
+
+ throw new StateError('More than two items is not supported: $parts');
+}
+
+/// Generates the MDN Comment for variables and method DOM elements.
+String _mdnMemberComment(String type, String member) {
+  var mdnType = _mdn[type];
+  if (mdnType == null) return '';
+  var mdnMember = mdnType['members'].firstWhere((e) => e['name'] == member,
+      orElse: () => null);
+  if (mdnMember == null) return '';
+  if (mdnMember['help'] == null || mdnMember['help'] == '') return '';
+  if (mdnMember['url'] == null) return '';
+  return _htmlifyMdn(mdnMember['help'], mdnMember['url']);
+}
+
+/// Generates the MDN Comment for class DOM elements.
+String _mdnTypeComment(String type) {
+  var mdnType = _mdn[type];
+  if (mdnType == null) return '';
+  if (mdnType['summary'] == null || mdnType['summary'] == "") return '';
+  if (mdnType['srcUrl'] == null) return '';
+  return _htmlifyMdn(mdnType['summary'], mdnType['srcUrl']);
+}
+
+/// Encloses the given content in an MDN div and the original source link.
+String _htmlifyMdn(String content, String url) {
+  return '<div class="mdn">' + content.trim() + '<p class="mdn-note">'
+      '<a href="' + url.trim() + '">from Mdn</a></p></div>';
+}