prohibit or at least warn about type parameter names shadowing existing type names

Newcomers to Dart sometimes end up writing code like this:

```
void foo(int x) {
}
```

where `int` in `foo` becomes a reference to type parameter, rather than just a reference to `int`.

This later might lead to all kinds of confusing things and errors like `int` is not a subtype of `int`.

We should consider either completely forbidding this or at least mandate a warning to make this new user friendly.

/cc @leafpetersen @lrhn @eernstg @munificent

Start by examining the Dart example `void foo(int x) {}` and the resulting type-resolution behavior. The issue is complete only after the language design chooses prohibition or warning behavior and specifies the corresponding user-facing result.

Written by the indexing model from the issue text.