Omitting an argument name should not turn into a implicit dynamic with an argument named after a type

Hi there, not sure if this belongs here or dart-lang/sdk.

It seems that abstract class method arguments work if they are `T`, `T argumentName`, but not `T`

```dart
abstract class Foo {
void valid(String);
void valid2(List foo);

void invalid(List); // Expected to find ')'
}
```

Reproduce the abstract-method examples from the issue and compare the behavior of String, List foo, and List in the Dart language specification. Identify the specification or grammar entry governing unnamed arguments, then confirm the intended result and add the corresponding coverage if the repository has a relevant test location.

Written by the indexing model from the issue text.