Specify behavior of external constructors with initializing formals

The specification for `external` functions is mostly grammatical. This prevents `external` functions from having bodies and `external` constructors from redirecting or having initializer lists, but doesn't account for `external` (non-redirecting, generative) constructors with initializing formals. Such constructors should be erroneous, but as far as I can tell, the type of error is never specified.

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For context, analyzer and CFE disagree on the following:
```dart
class Foo {
int x;
external Foo(this.x);
}
```
Analyzer finds no issues (and leaves it to backends to produce a runtime error), while CFE reports `Error: An external constructor can't initialize fields.`

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IMO this should at least be a compile-time error. The grammar could be rewritten to make this syntactically invalid, but that may not be worth it.