Requesting analyzer support for "trailing/closing tag" feature

[sethladd]

We quickly explored the usefulness of "closing tags" for Flutter code, and our initial users seem to find it helpful. We'd like to explore how to do this "for real" which we believe requires help/support from the analyzer, in order to improve robustness of the feature and help make it available in IntelliJ.

I'll defer to @DanTup for further explanation and technical requests.

[DanTup]

The current implementation linked above uses getHovers to retrieve offset/length information for the constructor calls. To know where constructors are it uses a combination of highlights and outlines. The whole thing is rather inefficient!

The idea is to implement a new notification in the analysis server that can provide all of the require info for rendering these decorations for an entire file in a single message - something like an array of int offset/int length/string label.

With assistance, I hope to contribute this feature :-)