Add support for mixins to Babel 6.x

[samwgoldman]

If this looks good I'll backport to Babel 5.x. In particular I'm unsure if it's kosher to reuse InterfaceExtends as the type of mixins nodes. Should I create a ClassMixins type instead?

cc @sebmck @jeffmo

[samwgoldman]

I forgot to add the necessary generator behaviors. Fixing now...

[samwgoldman]

OK, GTG.

[codecov-io]

Current coverage is 84.90%

Merging #3203 into master will decrease coverage by -0.19% as of 5cfec9d

@@            master   #3203   diff @@
======================================
  Files          215     215       
  Stmts        15617   15618     +1
  Branches      3341    3343     +2
  Methods          0       0       
======================================
- Hit          13289   13260    -29
- Partial        681     711    +30
  Missed        1647    1647       

Review entire Coverage Diff as of 5cfec9d

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[jamiebuilds]

All these changes to flow stuff need updates in:

• https://github.com/babel/babel/blob/master/packages/babel-types/src/definitions/flow.js
• babel-plugin-transform-flow-comments
• babel-plugin-transform-flow-strip-types

[sebmck]

We'll need a node.mixins existence check here too.

[samwgoldman]

@thejameskyle I think I addressed your comment, but can you confirm?

[sebmck]

There's still a missing mixins visitor key that will need to be added here.