Expose "why not promoted" information in VSCode

[stereotype441]

(Parent issue #44897)

As of a42244f, the analyzer now generates context messages in some circumstances explaining why type promotion failed. We would like these messages to show up in VSCode in a way that's easily discoverable.

[bwilkerson]

@DanTup

[devoncarew]

I filed Dart-Code/Dart-Code#3136 (and will close this issue in favor of that one).