Add the lint package

[passsy]

Enables a larger set of lint checks than the one which were already enabled by adding the lint package.

After enabling the new checks flutter analyze reports 317 errors and 1925 warnings which will help to improve the source code.

Most errors are caused by implicit-casts: false which requires manual casting from dynamic to the proper type during assignment. This will become the default soon and is therefore recommended to be fixed.

I recommend fixing the errors when merging this PR (nobody likes red squiggly lines everywhere) and then start fixing the warnings over time.

[hillelcoren]

Thanks for creating the PR!

My understanding is that when NNBD is released they're also going to provide automated tools to help migrate the code, do you think corrections for these errors/warnings will be included?

[passsy]

It seems likely that implicit cast will be automatically converted to explicit casts. May be @stereotype441 and @MichaelRFairhurst know.

Some of the remaining 2k warning can fixed relatively quick with IDE support (i.e. adding the const keyword)

But sadly there is not "Fix All" right now https://github.com/dart-lang/sdk/issues/37228