Try generating Dart bindings

Will have to figure out a nice way to implement operators, probably something like:

- `+` and `-` are always for two values of the same type
- `*` and `/` always take a plain `double` argument?
- `a.times(b)` means `b` is unitless, `a.product(b)` means `b` has the same units as `a`?
- `a.dot(b)`, `a.cross(b)` mean `b` is a unitless vector?
- can use `componentIn` for dot product with direction, or convert a direction to a unit vector
- `~/` and `%` for `Quantity` div/mod

Start by reviewing the operator questions listed in the issue and the existing approach to generated bindings. Determine how the proposed Dart operators should map to the listed quantity and vector operations. Done means the Dart binding approach and operator behavior are specified well enough to implement.

Written by the indexing model from the issue text.