Complex arithmetic and scalar operator overloading

The following features would make numerics in Dart much more pleasant

1. Complex type
2. Operator overloading with scalars

Kahan has some nice examples on pages 11 and 50
https://people.eecs.berkeley.edu/~wkahan/JAVAhurt.pdf

Start with the issue's two requested features—complex numbers and scalar operator overloading—and review the linked Kahan reference, especially pages 11 and 50. No source files or tests are identified; done would require a defined Dart language design and specification proposal for the requested behavior.

Written by the indexing model from the issue text.