Add equals and hashCode to Tween

[goderbauer]

Description

Adds an equals and hashCode method to Tween.

Related Issues

None

Tests

I added the following tests:

• test that equals work as expected

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
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Breaking Change

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• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.

[shihaohong]

LGTM

[Piinks]

LGTM!

[goderbauer]

Tree appears to be green, but dashboard is not updating. Submitting.

[Hixie]

@tvolkert @Piinks @shihaohong FYI this was reverted because it is unsound to put operator== on mutable objects and unsound for a hashCode to change over the lifetime of an object.