fix some warnings in generated code

[devoncarew]

Fix some warnings in generated code - only generate a private method if it's used in that class.

@adambender

[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/content_settings.dart, line 73, col 8)
[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/declarative_content.dart, line 30, col 8)
[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/declarative_web_request.dart, line 43, col 8)
[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/events.dart, line 23, col 8)
[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/extension_types.dart, line 23, col 8)
[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/privacy.dart, line 44, col 8)
[hint] The method '_throwNotAvailable' is not used (/Users/devoncarew/projects/chrome.dart/lib/gen/proxy.dart, line 40, col 8)

[adambender]

LGTM.