change DiagnosticableMixin to Diagnosticable

[quanghd96]

DiagnosticableMixin is removed on flutter repository (master branch). Now, Diagnosticable is a mixin on stable. So we need change DiagnosticableMixin to Diagnosticable to fix "Error: Type 'DiagnosticableMixin' not found".

[quanghd96]

@dylanwuzh is that ok?

[coleary01]

When will this likely be approved? It impacts on using "master" branch of Flutter.

[ymback]

Also Beta

[peterlpt]

现在在beta分支也有这个问题了，望 @dylanwuzh 早日合并此请求

[chandrabezzo]

@dylanwuzh In latest version of flutter, we need this change.

[guohuanle]

Now the version after 1.20 is not available. The temporary solution is to modify the source code and change "DiagnosticableMixin" to "DiagnosticableTreeMixin" so that both the new version and the old version are available. I don’t know if this modification is correct, I hope to update the version as soon as possible (some people say it can be changed to "Diagnosticable", but this only solves the problem of the new version of the SDK, the old version of the SDK is not compatible)

[quanghd96]

@guohuanle if release new version, someone use old SDK can use old version of this library.

[LugonjaAleksandar]

Latest Flutter 1.20.1 now also has this issue, and plugin is unusable. Someone has to take care of this and merge it, otherwise no app can compile with latest Flutter version. @dylanwuzh can you please review this change since you are the biggest contributor to this repo, or at least tag someone who would be able to merge it....?

[TetsuFe]

I made another PR #124 to follow https://github.com/dylanwuzh/flutter-cupertino-date-picker/pull/107#issuecomment-670340448 use DiagnosticableTreeMixin