REFACTOR: break down the logic into dedicated methods, tests cleaned up

lib/bowling_game.dart

@@ -1,35 +1,58 @@
 class BowlingGame {
+  static const _totalFrames = 10;
+  static const _allPins = 10;
+  static const _rollsInNormalFrame = 2;
+  static const _rollsInStrike = 1;
+
   final List<int> _rolls = [];
 
-  void roll(int pins) {
-    _rolls.add(pins);
-  }
+  void roll(int pins) => _rolls.add(pins);
 
   int score() {
-    int totalScore = 0;
+    int total = 0;
     int rollIndex = 0;
 
-    for (int frame = 0; frame < 10; frame++) {
+    for (int frame = 0; frame < _totalFrames; frame++) {
       if (_isStrike(rollIndex)) {
-        totalScore += 10 + _rolls[rollIndex + 1] + _rolls[rollIndex + 2];
-        rollIndex += 1;
+        total += _strikeScore(rollIndex);
+        rollIndex += _rollsInStrike;
       } else if (_isSpare(rollIndex)) {
-        totalScore += 10 + _rolls[rollIndex + 2];
-        rollIndex += 2;
+        total += _spareScore(rollIndex);
+        rollIndex += _rollsInNormalFrame;
       } else {
-        totalScore += _rolls[rollIndex] + _rolls[rollIndex + 1];
-        rollIndex += 2;
+        total += _normalScore(rollIndex);
+        rollIndex += _rollsInNormalFrame;
       }
     }
 
-    return totalScore;
+    return total;
+  }
+
+  int _strikeScore(int rollIndex) {
+    return _allPins + _nextTwoRollsBonus(rollIndex);
+  }
+
+  int _spareScore(int rollIndex) {
+    return _allPins + _nextRollBonus(rollIndex);
+  }
+
+  int _normalScore(int rollIndex) {
+    return _rolls[rollIndex] + _rolls[rollIndex + 1];
+  }
+
+  int _nextTwoRollsBonus(int rollIndex) {
+    return _rolls[rollIndex + 1] + _rolls[rollIndex + 2];
+  }
+
+  int _nextRollBonus(int rollIndex) {
+    return _rolls[rollIndex + 2];
   }
 
   bool _isSpare(int rollIndex) {
-    return _rolls[rollIndex] + _rolls[rollIndex + 1] == 10;
+    return _rolls[rollIndex] + _rolls[rollIndex + 1] == _allPins;
   }
 
   bool _isStrike(int rollIndex) {
-    return _rolls[rollIndex] == 10;
+    return _rolls[rollIndex] == _allPins;
   }
 }