update spec on evaluation order of deferred checks

I couldn't find in the spec a lot of details explaining deferred checks. The program below:

```dart
import 'a.dart' deferred as a;

main() {
var x = 1;
try {
a.foo(x = 2);
} finally {
print(x);
}
```

We will be injecting a check that "a" is loaded before we allow the call to "foo". This check can be injected before or after arguments are evaluated to a value. This is implemented differently in dart2js and the VM: the example above prints 1 in dart2js and 2 in the vm.

In Dart 1 I'd have said we should implement the VM semantics. However, In Dart2 I believe we want the dart2js semantics instead: this matches the left-to-right consistency rule that we are adopting for Dart 2. I'm about to implement that in the FE at this moment.

@leafpetersen @lrhn @floitschG - can you clarify? would it be possible to explicitly call this out in the spec?

Related issue dart-lang/sdk#27577

Start with the deferred-check evaluation example in this issue and review the existing specification text on deferred checks; the payload names no file or test. Compare the dart2js and VM outcomes described here, then resolve the intended Dart 2 ordering and document it explicitly in the spec, including whether argument assignment occurs before the load check.

Written by the indexing model from the issue text.