Bug fix: Properly disposed of Timer for Rotate, Scale, and Typer.

[awhitford]

Separate PRs were submitted for Fade and Typewriter -- the fix is the same. Impacts issue #119.

Pull Request Process

1. Ensure any install or build dependencies are removed before the end of the layer when doing a build.
2. Update the README.md if needed with details of changes to the interface, this includes new environment variables, exposed ports, useful file locations and container parameters.
3. Increase the version numbers in any examples files and the README.md to the new version that this Pull Request would represent. The versioning scheme we use is SemVer.
4. You may merge the Pull Request in once you have the sign-off of one developer, or if you do not have permission to do that, you may request the second reviewer to merge it for you.

[codecov]

Codecov Report

Merging #123 into master will decrease coverage by 0.45%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   68.63%   68.18%   -0.46%     
==========================================
  Files           8        8              
  Lines         695      704       +9     
==========================================
+ Hits          477      480       +3     
- Misses        218      224       +6     

Impacted Files	Coverage Δ
lib/src/rotate.dart	65.65% <33.33%> (-1.02%)	⬇️
lib/src/scale.dart	65.88% <33.33%> (-1.20%)	⬇️
lib/src/typer.dart	63.29% <33.33%> (-1.19%)	⬇️

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[aagarwal1012]

lgtm