Use pub.dev instead of pub.dartlang.org

[sidrao2006]

Description

Use pub.dev in place of pub.dartlang.org which is redirected to pub.dev

Checklist

• [x] I am willing to follow-up on review comments in a timely manner.

[coveralls]

Pull Request Test Coverage Report for Build 140

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.391%

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Totals
Change from base Build 139:	0.0%
Covered Lines:	85
Relevant Lines:	92

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💛 - Coveralls

[MichaelMarner]

Thanks for the contribution!