Is there a way to find declared source of predefined annotations?

[wurikiji]

const predefinedAnnotation = SomeAnnotation('this is predefined');

@predefinedAnnotation  // this is the first case
@SomeAnnotation('this is defined as it's needed') // this is the second case
class SomeClass {
}

In this case, I am currently getting a declaration by using below code.

// `element `is `ClassElement` of SomeClass
for (final annotations in element. metadata) {
  print(annotations.toSource());
}

In the first case (predefined annotation), it only prints predefinedAnnotation, the name of the constant variable.
In the second case, it prints what I want. The full code of @SomeAnnotation('this is defined as it's needed').

Is there a way to get the full code in case of predefinedAnnotation?

[natebosch]

The first step is to get a DartObject for the annotation. I think annotations.computConstantValue() works in most cases, but if the annotation is not resolvable it may return null. With the DartObject you can use a ConstantReader to read fields from it.
https://pub.dev/documentation/source_gen/latest/source_gen/ConstantReader-class.html

See also LibraryReader which has methods for finding annotated elements and creating a ConstantReader for the annotations.

https://pub.dev/documentation/source_gen/latest/source_gen/LibraryReader-class.html