Dynamic property access

There is currently no syntax in dart fr dynamic property access without the dart:mirrors library.
In javscript we can do something like obj[prop] to access a property dynamically. dart may also have a feature like this.

The issue names no files or tests. Start by reviewing Dart's existing property-access rules and the dart:mirrors limitation, then compare the requested behavior with JavaScript's obj[prop] syntax. Done requires a concrete language-design proposal with specified syntax and behavior.

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