Replace CustomPoint with extension methods on Point

[rorystephenson]

The CustomPoint class predates dart extension methods. The name is confusing (custom in what sense, and why) and in looking at the implementation it became clear that it was essentially just adding extension methods.

Changing to a Point will avoid conversions in plugins which deal with libraries that use Point.

Code will work as-is without changes with the exception of the subtraction operator. Previously CustomPoint - CustomPoint did not trigger analyzer warnings but would cause a runtime exception. This is now replaced by subtract() extension methods which allow subtracting num from a Point and int from Point.

---

Issue #1522 prompted me to look at this and I believe it resolves that issue (we no longer add methods which are not runtime-safe to Point and Point already documents the potential runtime error with the * operator).

Note that the vast majority of changes are just renaming CustomPoint to Point and importing it.

[JaffaKetchup]

LGTM, apart from these few nitpicks for consistency and to keep things concise.

[rorystephenson]

Nice catch on the redundant imports @JaffaKetchup !

First I started by removing the import 'dart:math' as math; imports but that didn't work well with some classes which had min/max getters in classes in some of the tile related code. So then I started removing the new import 'dart:math'; imports but this meant that Point was always prefixed with math which makes quite a mess. I think importing math with a prefix makes sense for methods like min/max and getters like pi but in the case of Point it is pretty unambiguous.

For that reason I've updated the imports to:

import 'dart:math' as math hide Point;
import 'dart:math' show Point;

WDYT?

[JaffaKetchup]

It's less than ideal, I'm sure you'll agree, but OK for now!

[JaffaKetchup]

LGTM I think. Would like another review though, if anyone can do it today!

[mootw]

lgtm