Intersection of abstract types have non-abstract constructors (#4559)

• What if one is abstract and one isn't?
• Runtime semantics could go either way really
• Just make it concrete

Inference from 'this' property assignments in constructors (#4955)

• First up: Should we do this for TypeScript?
  • Pro: Consistent with what we've done before
  • Con: Bifurcates the universe in a weird way
  • This is exactly analogous to our object literal behavior
  • What about adding an option for this?
    • An option to use the additional JavaScript inference?
  • This is dangerous for derived classes that don't intend to declare new properties
  • What is the ES6 migration strategy?
    • Only issue one error per class
    • Offer a refactoring
      • Implement refactorings in tsserver ASAP
    • Revisit when the ES8 class property syntax lands

JavaScript files in TS compilations (#4826)

• Should provided JS files be included in concatenated output?
  • Yes
• Should JS files be copied to the output folder?
  • Yes
• Should JS files written in ES6 be transpiled when targeting ES5?
  • Yes
  • We're in the downleveling business whether or not we want to be