Add toIndex method

can be used to get the correct index from coordinates passed in. Will only calculate the index, not take into consideration any grid size constraints etc.

lib/src/Grid.dart

@@ -37,7 +37,14 @@ class Grid<E> extends DelegatingList<E> {
 
   void set(int x, int y, E value) => setElement(x, y, value);
 
-  int toIndex(int x, int y) => y * width + x;
+  /// 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
+  /// constraints etc; use [get] for that (generally recommended).
+  int toIndex(int x, int y) => (x < 0 || y < 0)
+      ? throw RangeError("Coordinates for Grid Indexing must not be negative.")
+      : y * width + x;
 
   Point<int> toCoords(int index) => Point<int>(index % width, index ~/ width);
 }

test/src/grid_test.dart

@@ -3,7 +3,6 @@ import 'package:test/test.dart';
 
 @Tags(const ["nobrowser"])
 void main() {
-  Grid sut;
   group("Instantiation", () {
     List<String> l;
     setUp(() {
@@ -69,23 +68,30 @@ void main() {
       expect(sut.width, 3);
     }, tags: const ["happy"]);
   });
-  group("getter", () {
+  group("toIndex", () {
+    Grid sut;
     setUp(() {
       sut = Grid(3, 3);
-      sut.setAll(0, [
-        "Hey",
-        "you",
-        "me",
-        "together",
-        "Hello",
-        "World",
-        "I",
-        "am",
-        "ready."
-      ]);
     });
-    test("Engine can be instantiated without canvas", () {
-      expect(sut, isNot(throwsNoSuchMethodError));
+    test("throws RangeError on negative x argument", () {
+      expect(() => sut.toIndex(-1, 2), throwsA(isRangeError));
+    }, tags: const ["bad"]);
+    test("throws RangeError on negative y argument", () {
+      expect(() => sut.toIndex(2, -1), throwsA(isRangeError));
+    }, tags: const ["bad"]);
+    test("calculates correct index for first element", () {
+      expect(sut.toIndex(0, 0), equals(0));
+    }, tags: const ["happy"]);
+    test("calculates correct index for last element", () {
+      expect(sut.toIndex(2, 2), equals(8));
+    }, tags: const ["happy"]);
+    test("calculates correct index for element on first row", () {
+      expect(sut.toIndex(2, 0), equals(2));
+    }, tags: const ["happy"]);
+    test("calculates correct index for example element", () {
+      expect(sut.toIndex(1, 1), equals(4));
+    }, tags: const ["happy"]);
+  });
     });
   });
 }