When the common type of a shared property is not accessible within the library, attempt again with an ancestor type

[CodingSoot]

The feature introduced in #740 is great.

However, sometimes the common type might not be accessible in the freezed file, which causes errors.

For example :

// In a library which only exports A, B and C and Common1 but doesn't export Common2
class Common1 {}

class Common2 extends Common1 {}

class A extends Common2{}

class B extends Common2{}

// In a different library that imports the library above
@freezed
class MyUnion with _$MyUnion {
  const factory MyUnion.first(A param) = _First;

  const factory MyUnion.second(B param) = _Second;
}

The generated getter is Common2 get param => throw _privateConstructorUsedError;, with an error Undefined class 'Common2'.

What should instead happen is skip the common type Common2, and pick instead Common1 which is higher in the class hierarchy but is accessible.