REFACTOR: extract logic and remove loop

lib/roman_numerals.dart

@@ -1,17 +1,17 @@
 String integerToRoman(int number) {
-  var result = '';
-  for (int i = 0; i < number; i++) {
-    if (number == 5) {
-      result += 'V';
-      return result;
+  const romanSymbols = {5: 'V', 1: 'I'};
+
+  if (_isSubtractiveCase(number)) {
+    return romanSymbols[1]! + romanSymbols[5]!;
   }
 
-    if (number == 4) {
-      result += 'IV';
-      return result;
+  if (_canRepeatSymbol(number)) {
+    return romanSymbols[1]! * number;
   }
 
-    result += 'I';
-  }
-  return result;
+  return romanSymbols[number] ?? '';
 }
+
+bool _isSubtractiveCase(int number) => number == 4;
+
+bool _canRepeatSymbol(int number) => number < 4;

test/roman_numerals_test.dart

@@ -14,11 +14,11 @@ void main() {
     expect(roman_numerals.integerToRoman(3), 'III');
   });
 
-  test('converts 5 to V', () {
-    expect(roman_numerals.integerToRoman(5), 'V');
-  });
-
   test('converts 4 to IV', () {
     expect(roman_numerals.integerToRoman(4), 'IV');
   });
+
+  test('converts 5 to V', () {
+    expect(roman_numerals.integerToRoman(5), 'V');
+  });
 }