Support Synchronous evaluation of FutureOr

## Feature request

When we have a variable `FutureOr futureOr`, `await futureOr` should run synchronously when the variable is not a `Future` (see https://stackoverflow.com/q/59213196/6509751).

## Current situation

At the moment, you need to use the following code if you want to run the evaluation synchronously when possible:

```dart
if (futureOr is int)
print(futureOr + 9);
else
print((await futureOr) + 9);
```

Start by reading the requested FutureOr behavior and the linked Stack Overflow example. Compare the proposed synchronous evaluation with the current Dart language semantics and determine which specification or implementation areas would need design work. Done means reaching agreement on the behavior and documenting or implementing the language change.

Written by the indexing model from the issue text.