This case is to implement something similar to the Flutter Outline view from IntelliJ (note: this is still heavily in dev, using an experimental API and subject to change!)

• Implement basic tree
• Navigate to code on selection
• Stop outline refactor commands appearing in the palette
• Refactors on context menu (Allow fetching of all Flutter Outline refactors for a given file in a single request (or include in Outline) dart-lang/sdk#32462)
• Widget (Flutter) icons
• Tree nodes collapse when performing refactor
• Tooltips
• Add a preview flag
• Sorting of context menu is bad
• Require 1.20 analysis server
• Update to new ranges provided by server 1.20
• Stale data renders while waiting for updated data when switching file
• Highlight code for selected item (Expose API for highlighting a portion of the document (as used in document symbol list) microsoft/vscode#45059)
• Position code on screen better (editor.revealRange should ensure first line of range is always visible microsoft/vscode#45742)
• Reveal tree node as navigating code
• Clicking out of editor (eg into debug output) shouldn't hide Outline (debug window is unexpectedly considered a "text editor" microsoft/vscode#45188)
• Set unique IDs per node (globally unique) so updating code/running refactor keeps correct item selected
• Don't collapse nodes on clicking (TreeDataProvider: allow selecting a TreeItem without affecting its collapsibleState microsoft/vscode#34130)
• Show outline above deps (How can I control the ordering of explorer trees? microsoft/vscode#45053)
• Tree loses focus on click (so can't navigate with keyboard)
• Method/field icons? (Allow tree views to use icons for symbol kinds on their nodes microsoft/vscode#45057)
• Move to its own sidebar icon
• Remove enableProposedApi
• Doesn't unselect node when you move out of all elements (Allow programatically un-selecting an item in a contributed tree microsoft/vscode#48754)