v0.28.0

• Fix a crash when a Dart library doesn't have the .dart suffix (#1897)
• Fix a crash when a Dart file is loaded with an unresolvable
  prefix import in analyzer (#1896)
• Do not execute tools to generate documentation for non-canonical
  elements that have canonical counterparts. This eliminates
  "duplicate" tool runs that serve no purpose for generated docs
  or macro loading. (#1898)
• Fix a minor problem where invalid command line parameters could
  generate an ugly exception (#1895)
• Remove internal copy of mustache4dart and depend on the mustache
  package. Many minor changes to templating as a result. (#1894)
• Breaking change to warning handling. Many new command line options
  and dartdoc_options.yaml settings can constrain and configure
  how warnings are interpreted by dartdoc, and whether they are fatal.
  The old --show-warnings flag has been removed. (#1891, #1343, #1412, #1480)
• Fix a crash when loading README.md files that have invalid UTF-8.
  (#1890, #1889)
• Early implementation of experiment flags for Dart tools, based
  on the analyzer (#1884)