fix: Prevent web incompatibility due to import of dart:io

[Vinzent03]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

To get the current platform for custom headers dart:io is used. This causes no issues on web using flutter, but building a web app with solely Dart causes issues.

What is the new behavior?

Use conditional imports to only use dart:io when available.
I've also added tests to prevent this issue in the future.

Additional context

close #1219

[coveralls]

Pull Request Test Coverage Report for Build 17712639281

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 80.743%

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Totals
Change from base Build 17477396007:	0.01%
Covered Lines:	3107
Relevant Lines:	3848

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💛 - Coveralls