Enable 'as' and 'is' syntax for types available only at runtime

There may be some obscure reason that this is not a good idea or even possible but it would be nice to be able to do the following:
```
var string = 'hello';
var int = 1;
print(int is string.runtimeType);
```
Or be able to save a type in a variable and use it later:
```
var string = 'hello';
var int = 1;
var aType = string.runtimeType;
print(int is aType);
```
This produces a pretty silly error:
`Error: 'aType' isn't a type.`

Is there a difference between types and the [type class](https://api.dart.dev/stable/2.8.0/dart-core/Type-class.html)?

Start with the issue's runtimeType examples and the linked dart:core Type-class documentation. Investigate how Dart distinguishes compile-time types from Type values in `is` and `as` expressions, and determine the intended semantics for using a runtime type variable. Done means a documented language-design decision or proposal that resolves the requested behavior.

Written by the indexing model from the issue text.