Fix byte-compiling errors and warnings

[iinozemtsev]

• Remove references to dart-in-block-p (which was removed in 199709f)
• Wrap defun dart--identifier with eval-and-compile, as rx macro is executed during compilation
• Remove unused syntax variable from let block
• Add missing type to defcustom dart-font-lock-extra-types
• Wrap references to flycheck functions and variables with fboundp and boundp

Fixes #100

[bradyt]

Thank you!