Fix strong mode errors

[aaronlademann-wf]

These changes fix strong-mode errors seen when using the Dart Dev Compiler (and in the analyzer, when run with Dart SDK 1.24.1):

error: The argument type '(MouseEvent) → Null' can't be assigned to the parameter type '(Event) → void'

I also turned on a few memory-leak-related analyzer lints.

@marcojakob

[marcojakob]

Thank you @aaronlademann-wf. Version 0.4.0 is ready on pub: https://pub.dartlang.org/packages/dnd

[aaronlademann-wf]

Thanks for your prompt attention on this @marcojakob 💯