Update dependencies + analyzer changes

[Zazo032]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

This rolls minimum Dart SDK to the bundled with Flutter's latest stable version. Also, bumps pedantic and octo_image to latest versions. The update of octo_image is required for placeholders to not be wrongly placed.

💥 Does this PR introduce a breaking change?

From octo_image:

[0.3.0] - 2020-09-07 
Fixed bug when fitting placeholders or error widgets.
BREAKING: Doesn't automatically center placeholders and error widgets anymore.

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[renefloor]

Which analyzer change do you think helps with #463?

Btw, it makes it easier to review if you make a PR per proposed change.
The update of octo_image is a no-brainer for me, but I have to think about the changes proposed with the analyzer for example.

[Zazo032]

As pub analyzer may complain that pedantic was not using the latest available version, I updated it and fixed the new warnings. I removed the rules from analysis_options.yaml because I think they're already included by including pedantic rules (if I removed one that was not included in pedantic, please tell me). Also, added 5961d95#diff-d62227308ce5fdb544ee6b0a8b157a69R18 which may help with pub's error as per flutter/plugins#2658

[renefloor]

Thanks for explaining all you did!

[renefloor]

For some reason bitrise still doesn't accept the 'ignore' on the missing method:

Running analyze
$ flutter "analyze"
Running "flutter pub get" in git...                                 4.5s
Analyzing git...                                                
  error • The function 'webOnlyInstantiateImageCodecFromUrl' isn't defined • lib/src/image_provider/_load_async_web.dart:16:13 • undefined_function