Fixes https://github.com/dart-lang/sdk/issues/54038
The main feature is a small expansion to what is allowed in the `flutter / assets` field. We used to enforce that the `assets` field is a list, and that each value is a string. Now we enforce that:
* the `assets` field is a list
* an asset is either a string, or
* an asset is a map, with at least a 'path' key, with a string value.
This requires deprecating the `ASSET_NOT_STRING` code, and introducing
three others:
* ASSET_MISSING_PATH
* ASSET_NOT_STRING_OR_MAP
* ASSET_PATH_NOT_STRING
In addition I cleaned up a UX issue: Several codes referenced an 'asset' key, but it should be 'assets'. (In addition, the `ASSET_FIELD_NOT_LIST` code should be `ASSETS_FIELD_NOT_LIST` but I did not fix that.)
I also refactored all of the code to be more modern:
* Use `var` instead of types in declarations
* Short circuit error-and-return with `if (foo is! YamlMap)`. We didn't use to have promotion here, but this led to a wild amount of nesting (if, if, for, if, if, else, if!).
Change-Id: Ia489ea631f7d1027fdb84a9a682c831752c5835c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/338593
Reviewed-by: Marya Belanger <mbelanger@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Sam Rawlins
3b648e4635