Shorthand for creating constant identifier var (like an enum with a single value)

If I understood it correctly, this is what is being asked for in #183.

Sometimes I have required a constant value where I don’t care about the value, but I want it to be unique (as if it were a single value enum), so I create a var for the static type safety convenience. This has been useful for creating constant map keys or in #183 there is another use case. Currently this can be done either by creating an enum type (which can be a bit clumsy) or by defining a var:

`static var myIdentifier = Object()` (static when inside objects)

The drawback of this is that myIdentifier cannot be declared constant, it is a bit verbose and a prone to errors if forgetting to write `static`. It would be useful if it were possible to create constant identifiers like:

```dart
symbol id1, id2, id3;
Map map = {id1: 0};
foo(id1);
print(map[id3]); // prints '2'
...
foo(symbol id) {
map[id]++;
// id1 != id when symbol comes from the outside, only way that they can be equal is when foo(id1)
// is invoked from within foo
symbol id1;
map[id1] = 0;
map[id2] = map[id1] + 1;
map[id3] = map[id2] << 1;
}
```

`symbol` should be scoped as any var but it remains constant within the scope of a class or method.

I use the word symbol as an illustration, but I do not think it is appropriate as it conflicts with Symbol. When I required this feature I investigated and arrived to the Symbol type, however Symbol type didn’t make much sense to me, I think doing #mySymbol should just be a shorthand for creating a single word string, so `#mySymbol == 'mySymbol'`. I read a similar comment on #251. Would it be possible to change the Symbol behavior in future Dart releases?