Support for Dart enums

Issue: https://youtrack.jetbrains.com/issue/WEB-13931

First cut at enum support
  * parser updates
  * structure (view) support
  * basic doc support and syntax highlighting
  * resolution of enum definitions (and with it rename and find usages)
  * completions for enum definitions

TODO: 
  * resolution/find usages/completion for enum constant declarations
  * cleanup (notably enum reference resolution)
  * more tests?