GREEN: store rolls in a list and check for spare

lib/bowling_game.dart

@@ -1,11 +1,24 @@
 class BowlingGame {
-  int _score = 0;
+  final List<int> _rolls = [];
 
   void roll(int pins) {
-    _score += pins;
+    _rolls.add(pins);
   }
 
   int score() {
-    return _score;
+    int totalScore = 0;
+    int rollIndex = 0;
+
+    for (int frame = 0; frame < 10; frame++) {
+      if (_rolls[rollIndex] + _rolls[rollIndex + 1] == 10) {
+        totalScore += 10 + _rolls[rollIndex + 2];
+        rollIndex += 2;
+      } else {
+        totalScore += _rolls[rollIndex] + _rolls[rollIndex + 1];
+        rollIndex += 2;
+      }
+    }
+
+    return totalScore;
   }
 }