Update Dart documentation (2.13.0)

Author: simon04

lib/docs/filters/dart/clean_html.rb

@@ -36,6 +36,9 @@ def call
         # Remove external links
         css('#external-links').remove
 
+        # Remove button to https://dart.dev/null-safety
+        css('.feature-null-safety').remove
+
         # Properly format header in v2 docs
         nested_title = at_css('div > h1')
         unless nested_title.nil?

lib/docs/scrapers/dart.rb

@@ -21,7 +21,7 @@ class Dart < FileScraper
     HTML
 
     version '2' do
-      self.release = '2.12.2'
+      self.release = '2.13.0'
       self.base_url = "https://api.dart.dev/stable/#{release}/"
     end