Depend on stable flutter instead of master

[Renzo-Olivares]

This change fixes #29

Navigator 2.0 has been merged into the stable branch of Flutter and landed with version 1.22 this month, but the TransitionBuilderPage (used to build pages with custom transitions) has not landed on stable. Issue #29 points out that the build is broken. To reduce build breakages we should depend on stable. This change includes any overhead for the starter branch to depend on stable.

This change implements a CustomTransitionBuilderPage, as a temporary solution for TransitionBuilderPage not being on stable.

Code snippets in instructions should be updated to reflect new code. Instructions to be on master branch at the beginning of the codelab can also be removed/replaced with stable branch.

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[Renzo-Olivares]

@googlebot I signed it!

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[guidezpl]

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[perclasson]

LGTM