[analysis_server] Improve handling of const keywords for color picker

There were some inconsistencies with insertion of "const" keywords when selecting colors from the color picker (eg. https://github.com/Dart-Code/Dart-Code/issues/5068).

Some of this was because some tests were testing different their own ranges that didn't match the ones the server would have provided (which meant the replacements were different).

This updates the tests to ensure all tests are using ranges that the server would have produced, changes to ensure any existing const keywords are included in the color expression (so if in future we allowed selecting named colours, existing const constructor keywords would be correctly removed) and fixes up some places where we didn't actually produce Colors (assigning top-level color variables in an initializer and constant patterns) but had tests assuming we did (which failed with the new checks).

Fixes https://github.com/Dart-Code/Dart-Code/issues/5068

Change-Id: Id8814a426df4cbc350febabd11c963447c4b0e73
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/362801
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>