set example codes as dart code

[jspw]

I found it annoying to read the example dart codes in the documents as they were not formated as dart code so i have just set them as dart code in docs.

[googlebot]

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[jspw]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[stuartmorgan]

This project is no longer maintained, and cannot accept any PRs. Community members interested in collaborating on a community-maintained fork can coordinate in this issue.