[frontend_server] add API stability documentatioon

Document some of the API stability guarantees that are required for the command line
API of the frontend_server binary. This will allow package:test and the webdev CLI to
begin using the frontend_server

Change-Id: I4bf05757c527e491b1059489b5dda53dfda876a4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174840
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Jonah Williams <jonahwilliams@google.com>

Author: jonahwilliams

pkg/frontend_server/README.md

@@ -0,0 +1,19 @@
+The frontend_server package is used by both the flutter command line tool and the frontend_server_client package (used by webdev and package:test).
+
+## API Stability
+
+Changes to the command line API or behavior should be tested against the follwing test suites (in addition to normal HHH testing):
+  * flutter_tools: https://github.com/flutter/flutter/tree/master/packages/flutter_tools
+  * frontend_server_client: https://github.com/dart-lang/webdev/tree/master/frontend_server_client
+
+Otherwise these changes will need to be carefully coordinated with the flutter tooling and frontend_server_client teams.
+
+This API stability does not cover any of the source code APIs.
+
+
+### Stable subset
+
+* The frontend_server kernel compilation and expression evaluation for kernel should be considered "stable".
+* The frontend_server JavaScript compilation is semi-stable, but clients should anticipate coordinated breaking changes in the future.
+* The frontend_server JavaScript expression evaluation is experimental and is expected to change significantly from Dec 2020 through the end of 2021.
+* Specific flags like the --flutter-widget-cache may be added for experimentation and should not be considered stable.