NullPointerException can be renamed as NullReferenceException

[DartBot]

This issue was originally filed by @ahmetaa

---

Just a cosmetic issue;
NullPointerException is used a lot in Java but I think naming is not exactly correct. Since there are no "pointers" in Dart it may be better to rename it as NullReferenceException (as in C#)

[iposva-google]

Set owner to @gbracha.
Added Area-Language, Triaged labels.

[gbracha]

Added Accepted label.

[anders-sandholm]

Added this to the M1 milestone.

[gbracha]

This was misclassified as a defect. Reclassifying.

---

Removed this from the M1 milestone.
Removed Type-Defect label.
Added Type-Enhancement label.

[gbracha]

Added WontFix label.