Fix tests

lib/src/list.dart

@@ -77,13 +77,12 @@ class $List<E> extends _$DelegatingList<E> {
 
   /// Creates an unmodifiable list containing all [elements].
   ///
-  /// The [Iterator] of [elements] provides the order of the elements.
-  ///
   /// An unmodifiable list cannot have its length or elements changed.
   /// If the elements are themselves immutable, then the resulting list
   /// is also immutable.
-  factory $List.unmodifiable(Iterable<E> elements) {
-    return $List(List.unmodifiable(elements));
+  factory $List.unmodifiable([Iterable<E> elements]) {
+    var elementsNotNull = elements ?? List();
+    return $List(List.unmodifiable(elementsNotNull));
   }
 
   set lastIndex(int value) => length = value + 1;

test/list_test.dart

@@ -43,16 +43,40 @@ void main() {
     expect($List<int>.generate(5, (i) => i), [0, 1, 2, 3, 4]);
   });
 
+  test("test List.unmodifiable()", () {
+    {
+      expect($List<int>.unmodifiable(), empty);
+    }
+    {
+      var list = $List<int>.unmodifiable([0, 1, 2]);
+      expect(list, [0, 1, 2]);
+      expect(() => list[0] = 2, throwsUnsupportedError);
+      expect(() => list.add(3), throwsUnsupportedError);
+      expect(() => list.clear(), throwsUnsupportedError);
+    }
+  });
+
   test("test lastIndex", () {
     expect(empty.lastIndex, -1);
     expect($List<int>.withLength(10).lastIndex, 9);
   });
 
-  test("test second third fourth", () {
-    var list = $([0, 1, 2, 3]);
-    expect(list.second, 1);
-    expect(list.third, 2);
-    expect(list.fourth, 3);
+  test("test set second third fourth", () {
+    {
+      var list = $([0, 1, 2, 3]);
+      list.second = 8;
+      expect(list.second, 8);
+    }
+    {
+      var list = $([0, 1, 2, 3]);
+      list.third = 5;
+      expect(list.third, 5);
+    }
+    {
+      var list = $([0, 1, 2, 3]);
+      list.fourth = 1;
+      expect(list.fourth, 1);
+    }
   });
 
   test("test get", () {
@@ -113,6 +137,9 @@ void main() {
       var result = elements.dropLast(10);
       expect(result, <int>[]);
     }
+    {
+      expect(() => elements.dropLast(-1), throwsArgumentError);
+    }
   });
 
   test("test dropLastWhile", () {