check-unused-code should find unused class methods

[timukasr]

Description
Not sure if bug or missing feature, but check-unused-code command is unable to find unused class methods. Ideally, it should detect following cases:

• unused private and public methods
• public methods that are only used inside the class/file (so these could be private)

Why do you consider it necessary?
Quite often, when refactoring code, unused methods remain in class. IDEs like Android Studio are able to detect unused private methods, but not unused public methods. Also, the documentation says:

Checks unused classes, fields, properties, methods, functions, variables, extensions, enums, mixins and type aliases.

[app-toolkit-opensource-innersource]

Assign to review: @santitigaga @ajtortolero