Adds sample code for RaisedButton widget

[laertispappas]

Addresses #12382 and #21136

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[laertispappas]

I signed it!

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CLAs look good, thanks!

[Hixie]

nit: extraneous blank line

[Hixie]

nit: also here

[Hixie]

This looks great, just some minor nits!

When you update this you'll probably find it complains about MyHomePage being already used. You'll need to change the class name to be different than in any other example, e.g. CounterPage or something.

[laertispappas]

@Hixie thanks for the feedback. I will follow up.

[laertispappas]

@Hixie I have updated the code. Wanna have alook? Thank you in advance.

[HansMuller]

Please accept my apologies for not getting your contribution landed. I'm afraid it got lost in the final push for Flutter 1.0. That's no excuse really, sorry about dropping it.

Since then another developer has contributed a RaisedButton sample, #27169, which will serve as well.

I'm going to close this PR. I hope this will not discourage you from future contributions!

[laertispappas]

@HansMuller thanks for the update and your message. It's okay I understand. I hope I get more involved in the near future.