adding clipboard support for git clone dialog

@gaurave PR for https://github.com/dart-lang/spark/issues/2235 


`r"([\w\.@\:/\-~]+)(\.git)(/)?")` => `"([\w\.@\:/\-~]+)(\.git)(/)?")`. The `r` breaks the regex.

Can you make this a top level const  `GIT_URL_PATTERN`. This may be useful elsewhere.