pkg/csslib: types, fixes, cleanup

pkg/csslib/test/debug_test.dart

+library debug;
+
+import 'package:unittest/unittest.dart';
+import 'testing.dart';
+
+void main() {
+  test("excercise debug", () {
+    var style = parseCss(_input);
+
+    var debugValue = style.toDebugString();
+    expect(debugValue, isNotNull);
+
+    var style2 = style.clone();
+
+    expect(style2.toDebugString(), debugValue);
+  });
+}
+
+const String _input = r'''
+.foo {
+background-color: #191919;
+width: 10PX;
+height: 22mM !important;
+border-width: 20cm;
+margin-width: 33%;
+border-height: 30EM;
+width: .6in;
+length: 1.2in;
+-web-stuff: -10Px;
+}''';