Abstract modifier unnecessary for abstract methods

[gbracha]

An abstract method (or getter/setter) need not have an abstract modifier; the absence of a body unambiguously identifies it as abstract.

By making the abstract modifier optional or eliminating it entirely, one can encourage a pattern where abstract classes are used instead of interfaces with little syntactic penalty.

This is already in the 0.11 draft spec.

[gbracha]

Analyzer issue #3899
VM issue #3900
dart2js issue #3901

This would be the first step on the path to eliminating interface declarations. Then, most interfaces can be replaced with abstract classes. At the same time, implement redirecting constructors, allowing the remaining interfaces to be converted. Then, support for the interface syntax can be removed.