Add .get method for coordinate element retrieval to grid

Coordinates passed in access the correct index in the internal list.

lib/src/Grid.dart

@@ -21,10 +21,10 @@ class Grid<E> extends DelegatingList<E> {
         height = h,
         super(l);
 
-  E elementAtPos(int x, int y) {
+  E get(int x, int y) {
     int i = toIndex(x, y);
-    if (i >= length) throw RangeError.index(i, this);
-    _internal[i];
+    if (i >= length || x > width - 1) throw RangeError.index(i, this);
+    return _internal[i];
   }
 
   void setElement(int x, int y, E value) {
@@ -33,8 +33,6 @@ class Grid<E> extends DelegatingList<E> {
     _internal[i] = value;
   }
 
-  E get(int x, int y) => elementAtPos(x, y);
-
   void set(int x, int y, E value) => setElement(x, y, value);
 
   /// Calculate list index from coordinates.

test/src/grid_test.dart

@@ -92,6 +92,27 @@ void main() {
       expect(sut.toIndex(1, 1), equals(4));
     }, tags: const ["happy"]);
   });
+  group("coordinates getter", () {
+    Grid sut;
+    setUp(() {
+      sut = Grid(3, 3);
+      sut.setAll(0,
+          ["Hey", "you", "me", "together", "Hello", null, "I", "am", "ready."]);
     });
+    test("returns null if no element exists at the position requested", () {
+      expect(sut.get(2, 1), null);
+    }, tags: const ["sad"]);
+    test("throws RangeError if requesting element outside of grid width", () {
+      expect(() => sut.get(4, 1), throwsRangeError);
+    }, tags: const ["bad"]);
+    test("throws RangeError if requesting element outside of grid height", () {
+      expect(() => sut.get(1, 4), throwsRangeError);
+    }, tags: const ["bad"]);
+    test("returns element at correct index", () {
+      expect(sut.get(1, 0), "you");
+    }, tags: const ["happy"]);
+    test("returns last element correctly", () {
+      expect(sut.get(2, 2), "ready.");
+    }, tags: const ["happy"]);
   });
 }