need to report location of duplicate definition

[DartBot]

This issue was originally filed by [email protected]

---

Currently, in frog and in the vm, if a class is declared twice, the error reported does not include the location of the duplicate.

For example:

class Foo {
  int a;
}

class Foo {
  int b;
}

frog currently just says:
bug1.dart:5:1 7: 'warning: duplicate definition of Foo'

The error should report what the duplicate class is.

[DartBot]

This comment was originally written by [email protected]

---

Added Area-Frog, Triaged labels.

[jmesserly]

One line fix incoming :)

---

Set owner to @jmesserly.
Added Started label.

[DartBot]

This comment was originally written by [email protected]

---

Thanks, very helpful.

[jmesserly]

Added Fixed label.