Provide documentation around `--define` arguments

Our [docs for how to debug with dart2js](https://github.com/dart-lang/build/blob/master/docs/faq.md#how-can-i-debug-my-release-mode-web-app-dart2js) point to using `--define` but that doesn't work with `webdev`.

For now you can workaround with `pub run build_runner serve` instead of `webdev` but that option will go away eventually.

We either need to update the docs, or support the argument.

Start with docs/faq.md and its dart2js debugging section, then inspect how webdev handles the --define argument compared with pub run build_runner serve. Done means either documenting a supported webdev workflow or establishing support for the argument, with the eventual removal of the workaround addressed.

Written by the indexing model from the issue text.