Add Coordinates setter

Sets the corresponding element to the parameter value passed in. Checks against the grid size constraints beforehand and throws RangeError if outside of constraints. Preferred method to set
element via coordinates.

lib/src/Grid.dart

@@ -33,14 +33,18 @@ class Grid<E> extends DelegatingList<E> {
     return _internal[i];
   }
 
-  void setElement(int x, int y, E value) {
+  /// Sets element at coordinate position
+  ///
+  /// Sets the corresponding element to the [E] parameter [value] passed in.
+  /// Checks against the grid size constraints beforehand and throws
+  /// [RangeError] if outside of constraints. Preferred method to set
+  /// elements via coordinates.
+  void set(int x, int y, E value) {
     int i = toIndex(x, y);
-    if (i >= length) throw RangeError.index(i, this);
+    if (i >= length || x > width - 1) throw RangeError.index(i, this);
     _internal[i] = value;
   }
 
-  void set(int x, int y, E value) => setElement(x, y, value);
-
   /// Calculate list index from coordinates
   ///
   /// Can be used to get the correct index from coordinates passed in.

test/src/grid_test.dart

@@ -138,4 +138,30 @@ void main() {
       expect(sut.toCoordinates(6), equals(Point(0, 2)));
     }, tags: const ["happy"]);
   });
+  group("coordinates setter", () {
+    Grid<String> sut;
+    setUp(() {
+      sut = Grid(3, 3);
+      sut.setAll(0,
+          ["Hey", "you", "me", "together", "Hello", null, "I", "am", "ready."]);
+    });
+    test("sets element to null if passing null in", () {
+      sut.set(1, 1, null);
+      expect(sut.get(1, 1), null);
+    }, tags: const ["sad"]);
+    test("throws RangeError if setting element outside of grid width", () {
+      expect(() => sut.set(4, 1, "testValue"), throwsRangeError);
+    }, tags: const ["bad"]);
+    test("throws RangeError if setting element outside of grid height", () {
+      expect(() => sut.set(1, 4, "testValue"), throwsRangeError);
+    }, tags: const ["bad"]);
+    test("sets element at correct index", () {
+      sut.set(1, 0, "testValue");
+      expect(sut.get(1, 0), "testValue");
+    }, tags: const ["happy"]);
+    test("sets last element correctly", () {
+      sut.set(2, 2, "testValue");
+      expect(sut.get(2, 2), "testValue");
+    }, tags: const ["happy"]);
+  });
 }