fix: Use enum instead of const int internally in length.dart

lib/src/style/length.dart

@@ -1,55 +1,62 @@
-/// Increase new base unit types' values by a factor of 2 each time.
-const int _percent = 0x1;
-const int _length = 0x2;
-const int _auto = 0x4;
+/// These are the base unit types
+enum _UnitType {
+  percent,
+  length,
+  auto,
+  lengthPercent(children: [_UnitType.length, _UnitType.percent]),
+  lengthPercentAuto(children: [_UnitType.length, _UnitType.percent, _UnitType.auto]);
 
-/// These values are combinations of the base unit-types
-const int _lengthPercent = _length | _percent;
-const int _lengthPercentAuto = _lengthPercent | _auto;
+  final List<_UnitType> children;
+
+  const _UnitType({this.children = const []});
+
+  bool matches(_UnitType other) {
+    return this == other || children.contains(other);
+  }
+}
 
 /// A Unit represents a CSS unit
 enum Unit {
   //ch,
-  em(_length),
+  em(_UnitType.length),
   //ex,
-  percent(_percent),
-  px(_length),
-  rem(_length),
+  percent(_UnitType.percent),
+  px(_UnitType.length),
+  rem(_UnitType.length),
   //Q,
   //vh,
   //vw,
-  auto(_auto);
+  auto(_UnitType.auto);
 
   const Unit(this.unitType);
-  final int unitType;
+  final _UnitType unitType;
 }
 
 /// Represents a CSS dimension https://drafts.csswg.org/css-values/#dimensions
 abstract class Dimension {
   double value;
   Unit unit;
 
-  Dimension(this.value, this.unit, int _dimensionUnitType)
-      : assert(
-            identical((unit.unitType | _dimensionUnitType), _dimensionUnitType),
-            "This dimension was given a Unit that isn't specified.");
+  Dimension(this.value, this.unit, _UnitType _dimensionUnitType)
+      : assert(_dimensionUnitType.matches(unit.unitType),
+            "This Dimension was given a Unit that isn't specified.");
 }
 
 /// This dimension takes a value with a length unit such as px or em. Note that
 /// these can be fixed or relative (but they must not be a percent)
 class Length extends Dimension {
-  Length(double value, [Unit unit = Unit.px]) : super(value, unit, _length);
+  Length(double value, [Unit unit = Unit.px]) : super(value, unit, _UnitType.length);
 }
 
 /// This dimension takes a value with a length-percent unit such as px or em
 /// or %. Note that these can be fixed or relative (but they must not be a
 /// percent)
 class LengthOrPercent extends Dimension {
   LengthOrPercent(double value, [Unit unit = Unit.px])
-      : super(value, unit, _lengthPercent);
+      : super(value, unit, _UnitType.lengthPercent);
 }
 
 class AutoOrLengthOrPercent extends Dimension {
   AutoOrLengthOrPercent(double value, [Unit unit = Unit.px])
-      : super(value, unit, _lengthPercentAuto);
+      : super(value, unit, _UnitType.lengthPercentAuto);
 }