[dartpy] Add constraint APIs

[jslee02]

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Before creating a pull request

• Document new methods and classes
• Format new code files using clang-format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[codecov]

Codecov Report

Merging #1333 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1333   +/-   ##
=======================================
  Coverage   56.89%   56.89%           
=======================================
  Files         366      366           
  Lines       27167    27167           
=======================================
  Hits        15457    15457           
  Misses      11710    11710