While walking through the GeneratorAdapter code, I noticed a few oddities. While tidying things up, I found that each of the subclasses could have const constructors, which I think are preferred by the Flutter team.

• The parent class, GeneratorAdapter, had a fileTypeList field which never stored a value. Each subclass implements the parent class, so the constructor was never called. I changed the field to a getter (for the interface), and removed the constructor.
• Some adapters had a mutable instance field, String languageString which could be made static and const.
• In each adapter subclass, I made the List<FileType> fileTypeList into a getter, to avoid the "Fields in const classes should not have initializers" enforced lint rule.
• Some of the adapter subclasses had this.fileTypeList as a constructor parameter, but a value was essentially never passed in. (There was once instance in a test, but it had no effect.) So I removed this constructor parameter, moving the const default values to the getter value.
• Then each constructor can be made const.
• The implementation of _openSink had some unnecessary statements, so I made it more concise. The sink variable was unnecessary, and the file variable was only used in one else-branch, and so should be declared in that block.

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