Implement the Q1 2021 small features release in IntelliJ

[leafpetersen]

Implementation issue for this set of features.

cc @devoncarew @jwren @franklinyow

[devoncarew]

@jwren - the feature spec is here: https://github.com/dart-lang/language/tree/master/accepted/future-releases/small-features-21Q1

[jwren]

Thanks Leaf, this was not forgotten. When I sync with Alex at JetBrains next I'll make sure that he is iterating on this, otherwise I'll make sure it get into the Dart Plugin.

[jwren]

@leafpetersen @devoncarew

Allow type arguments on annotations (#1297).

Implemented with JetBrains/intellij-plugins#794, not yet landed at this point.

Allow generic function types as type arguments and bounds (#496).

There doesn't appear to be any changes required in the Dart Plugin PSI parser for this change.

Allow >>> as an overridable operator (#120).

Implemented with JetBrains/intellij-plugins#792, this change did land in the master branch last Friday.

Alexander is aware that we'll need these pushed into older versions of IDEA platform.

[jwren]

All PRs landed, closing issue.