Support compile-time source & bytecode aspect-oriented programming, similar to AspectJ

It's now 2021, and Dart is exploding in popularity, for good reason. I've worked with many languages in my 30+ years as a hands-on dev, and, after having leveraged the power of AspectJ for a _long_ time, I honestly can't bear life without it. I've even created a [decorator-driven AOP implementation for JavaScript](https://www.npmjs.com/package/@northscaler/aspectify) that works great, especially when combined with [continuation-local storage](https://www.npmjs.com/package/@northscaler/continuation-local-storage) (like Dart zones). Dart has _so many_ great features IMHO. The only one that's missing for me is AOP support built into the language & compiler. The feature set I'd like would be the same as AspectJ's, possibly without intertype declarations (ITDs, which I use to implement a mixin/trait pattern), because Dart supports mixins natively.

Dart is _so_ promising. Please add native AOP at the language & compiler level!

No files, tests, or entry points are named. Start by reviewing the requested AspectJ-like feature set and the Dart language and compiler design; define the supported AOP semantics and scope before identifying affected specification and implementation areas. Done would require an agreed design and corresponding language and compiler changes.

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