Add Grid.fill constructor

Will completely fill the grid with the value passed in.

lib/src/Grid.dart

@@ -10,6 +10,9 @@ class Grid<E> extends DelegatingList<E> {
 
   Grid(int width, int height) : this._(List<E>(width * height), width, height);
 
+  Grid.fill(int width, int height, E fillValue)
+      : this._(List<E>.filled(width * height, fillValue), width, height);
+
   Grid.from(Grid<E> l)
       : this._(List<E>.from(l.getRange(0, l.length)), l.width, l.height);
 

test/src/grid_test.dart

@@ -73,6 +73,24 @@ void main() {
         expect(sut.width, 3);
       }, tags: const ["happy"]);
     });
+    group(".fill", () {
+      test("fills list with results of function passed in", () {
+        Grid<String> sut = Grid.fill(3, 3, "testValue");
+        expect(
+            sut,
+            containsAllInOrder([
+              "testValue",
+              "testValue",
+              "testValue",
+              "testValue",
+              "testValue",
+              "testValue",
+              "testValue",
+              "testValue",
+              "testValue"
+            ]));
+      }, tags: const ["happy"]);
+    });
   });
   group("toIndex", () {
     Grid sut;