Proposal: operator !! (opposite of ??)

Proposed solution to #360.

Where `a ?? b` is sugar for `a != null ? a : b`, there may be value in adding `!!` defined as `a == null ? null : b`.

Example: calling a function when the parameter is not null.

```dart
String? toPrint;
toPrint !! print(toPrint);
```

Example: awaiting a future when its not null

```dart
Future? fut;
fut !! await fut;
```

Example: asserting an object is fully initialized

```dart
assert(name !! age !! email !! password != null)
```

Similar to #219 except I think it has a few different advantages:

- extremely simple and consistent with an existing well-used and understood dart concept
- binary operator so left-to-right flow is easier to follow
- wouldn't interact with any other syntax (prefix ? is was proposed to interact with maps, lists, optional parameters, etc.)

Disadvantages:

- using `!` and not throwing an exception on null is confusing. Maybe `?!` or `!?`?
- Limited usefulness
- repeat lhs required to take advantage of promotion

Read this proposal, then follow #360 and #219 and the 18-comment thread to understand the unresolved syntax and usefulness questions. No implementation files or tests are named; the work is only ready when the language design is decided and its specification and implementation touchpoints are identified.

Written by the indexing model from the issue text.