[Class modifiers] Analysis server / IDE support work

[itsjustkevin]

This is the issue tracking the work that needs to be done in the analysis server to support the class modifiers feature. If you decide to work on a feature, either create an issue for the feature and link to it from here, or put your name beside the feature so that we don't duplicate effort.

• Call Hierarchy
  (brianwilkerson, NA)
• Closing Labels
  (brianwilkerson, NA)
• Code Completion
  Need to suggest the new modifiers and filter out suggestions that are invalid based on class capabilities
• Code Folding
  (brianwilkerson, NA)
• documentSymbol
  (brianwilkerson, NA)
• Flutter Outline
  (brianwilkerson, NA)
• Hovers
  Need to add the new modifiers to the hover help
• Implemented Markers
  (brianwilkerson, NA)
• Navigation
  (brianwilkerson, NA)
• Occurrences
  (brianwilkerson, NA)
• Organize Imports
  (brianwilkerson, NA)
• Outline
  NA
• Overrides Markers
  (brianwilkerson, NA)
• Quick Assists for Class Modifiers #51496
  Needs review
• Quick Fixes for Class Modifier Diagnostics #51440
  (@pq)
• Refactorings - legacy
  (brianwilkerson, NA)
• Refactorings - self describing
  (brianwilkerson, NA)
• Search - Find References
  (brianwilkerson, NA)
• Search - Member Declarations
  (brianwilkerson, NA)
• Search - Member References
  (brianwilkerson, NA)
• Search - Top-level Declarations
  (brianwilkerson, NA)
• selectionRange
  (brianwilkerson, NA)
• Semantic Highlights
  Highlight new keywords
• signatureHelp
  (brianwilkerson, NA)
• Snippets
  (brianwilkerson, NA)
• Sort Members
  (brianwilkerson, NA)
• Type Hierarchy
  (brianwilkerson, NA)

[pq]

We're pushing Class Modifier quick-fixes until post-beta (pending further investigation into what we want to support) so I think this issue is good to close as complete.