Add .toCoordinates() method to grid

Calculates the 2-D array coordinates from the corresponding list index passed in. Relies on grid width to calculate coordinates. Does not check against grid size constraints.

lib/src/Grid.dart

@@ -35,7 +35,7 @@ class Grid<E> extends DelegatingList<E> {
 
   void set(int x, int y, E value) => setElement(x, y, value);
 
-  /// Calculate list index from coordinates.
+  /// Calculate list index from coordinates
   ///
   /// Can be used to get the correct index from coordinates passed in.
   /// Will only calculate the index, not take into consideration any grid size
@@ -44,5 +44,12 @@ class Grid<E> extends DelegatingList<E> {
       ? throw RangeError("Coordinates for Grid Indexing must not be negative.")
       : y * width + x;
 
-  Point<int> toCoords(int index) => Point<int>(index % width, index ~/ width);
+  /// Calculate coordinates from list index
+  ///
+  /// Calculates the 2-D array coordinates from the corresponding list index
+  /// passed in. Relies on grid width to calculate coordinates. Does not check
+  /// against grid size constraints; use [set] for that (generally recommended).
+  Point<int> toCoordinates(int index) => (index < 0)
+      ? throw RangeError("Index for Grid Coordinates must not be negative")
+      : Point<int>(index % width, index ~/ width);
 }

test/src/grid_test.dart

@@ -1,3 +1,5 @@
+import 'dart:math';
+
 import 'package:rules_of_living/src/Grid.dart';
 import 'package:test/test.dart';
 
@@ -115,4 +117,25 @@ void main() {
       expect(sut.get(2, 2), "ready.");
     }, tags: const ["happy"]);
   });
+  group("toCoords", () {
+    Grid sut;
+    setUp(() {
+      sut = Grid(3, 3);
+    });
+    test("throws RangeError on negative index argument", () {
+      expect(() => sut.toCoordinates(-1), throwsA(isRangeError));
+    }, tags: const ["bad"]);
+    test("calculates correct index for first element", () {
+      expect(sut.toCoordinates(0), equals(Point(0, 0)));
+    }, tags: const ["happy"]);
+    test("calculates correct index for last element", () {
+      expect(sut.toCoordinates(8), equals(Point(2, 2)));
+    }, tags: const ["happy"]);
+    test("calculates correct index for last element on first row", () {
+      expect(sut.toCoordinates(2), equals(Point(2, 0)));
+    }, tags: const ["happy"]);
+    test("calculates correct index for example element", () {
+      expect(sut.toCoordinates(6), equals(Point(0, 2)));
+    }, tags: const ["happy"]);
+  });
 }