Fix linter warnings

[Ephenodrom]

I fixed all linter warnings

• Only use double quotes for strings containing single quotes.
• Unnecessary new keyword.
• Omit type annotations for local variables.
• Annotate overridden members.

In the following folders :

• benchmark
• tutorials
• test

This is a first step to fix all linter warnings as suggested in the following issue

https://github.com/bcgit/pc-dart/issues/16

Output of unit tests

pub run test
00:51 +361: All tests passed!                                               

[AKushWarrior]

@mwcw this is kind of important for release. Can you merge it?

[mwcw]

Hi,

Thank you very much for fixing those linter issues.

It will be synced up shortly.

Thanks,

MW

[Ephenodrom]

Thanks for accepting the PR, will try to fix more linter warnings throughout the next weeks.