"if not null" operators

Dart supports "if null" access and assignment operators — `??`, `??=`.

Equivalent "if not null" operators would also be useful, particularly when non-nullable types land (https://github.com/dart-lang/language/issues/110). They would allow concise use of nullable variables in a similar manner to quiver's `Optional`. E.g., instead of:

```
Optional foo;
foo = foo.transform(changeFoo);
final bar = foo.transform(fooToBar);
```

nullable variables could be used:

```
Foo? foo;
foo ?!= changeFoo(foo);
final bar = foo ?! fooToBar(foo);
```

Start with the proposed `?!` and `?!=` examples in this issue, then read language issue #110 for the non-nullable-type context. Compare the requested behavior with Dart's existing `??` and `??=` operators and determine whether the syntax and semantics can be specified. Done means the language-design question has an agreed decision.

Written by the indexing model from the issue text.