fix: mixins are not ClassElement

[genesistms]

This PR solves the issue but i am not sure if this is correct approach. I would like to add tests if someone could point me where would be the best place.

I will mention @ykmnkmi as this is mostly his work.

When compiling this:

@Directive(selector: 'dir')
class MyDir with MyMixin {}

mixin MyMixin {}

you get this error:

type 'MixinElementImpl' is not a subtype of type 'ClassElement' in type cast.

[GZGavinZhao]

@genesistms You can add the tests in the file _tests/test/compiler_integration/invalid_component_test.dart using the compilesNormally helper function, like this line.

[genesistms]

Ok i added a one simple test for this use case. After resolving this issue i will try to add more similar to this.