Update dartdoc to 0.13.0+1 for flutter (#10863)

dev/bots/docs.sh

@@ -8,7 +8,7 @@ set -e
 bin/flutter --version
 
 # Install dartdoc.
-bin/cache/dart-sdk/bin/pub global activate dartdoc 0.12.0
+bin/cache/dart-sdk/bin/pub global activate dartdoc 0.13.0+1
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.

dev/tools/dartdoc.dart

@@ -35,6 +35,7 @@ Future<Null> main(List<String> args) async {
   // Create the pubspec.yaml file.
   final StringBuffer buf = new StringBuffer('''
 name: Flutter
+homepage: https://flutter.io
 version: $version
 dependencies:
 ''');