No static error when iterating over a generic type, T.

@matanlurey commented on [Thu May 30 2019](https://github.com/dart-lang/sdk/issues/37125)

```dart
void main() {
example1(5);
example2(3);
}

void example1(T item) {
// No static error.
// Runtime error (in Dart2JS):
// TypeError: 5: type 'JSInt' is not a subtype of type 'Iterable'
for (final x in item) {
print(x);
}
}

void example2(T item) {
// Static error: The method 'iterator' isn't defined for the class 'Object'.
final iterator = item.iterator();
while (iterator.moveNext()) {
print(iterator.current);
}
}
```

I'd expect the same or similar error in both cases.

/cc @leafpetersen

---

@vsmenon commented on [Thu May 30 2019](https://github.com/dart-lang/sdk/issues/37125#issuecomment-497520142)

@leafpetersen @lrhn - is this a language issue or an implementation one? Right now, it looks like none of the implementations report a static error for the `for`.

Start with the Dart examples in the issue and compare the analyzer's handling of `for (final x in item)` with the explicit `item.iterator()` call. Review the language specification and implementation behavior to determine whether both cases should produce a static error. Done means the intended behavior is specified and the relevant implementations consistently enforce it.

Written by the indexing model from the issue text.