Fix bugs in ListExtensions.slice and ListSlice.slice. (#290)

Thanks to [@hcanyz](https://github.com/hcanyz) for finding the bugs,
and suggesting fixes.
Adds regression test.

Fixes #286

CHANGELOG.md

@@ -5,6 +5,7 @@
     - `copy`: copies an instance without recalculating the canonical values of the keys.
     - `toMap`: creates a `Map<K,V>` (with the original key values).
     - `toMapOfCanonicalKeys`: creates a `Map<C,V>` (with the canonicalized keys).
+- Fixes bugs in `ListSlice.slice` and `ListExtensions.slice`.
 - lints: ^2.0.1
 
 ## 1.17.2

lib/src/list_extensions.dart

@@ -241,7 +241,7 @@ extension ListExtensions<E> on List<E> {
   ListSlice<E> slice(int start, [int? end]) {
     end = RangeError.checkValidRange(start, end, length);
     var self = this;
-    if (self is ListSlice) return self.slice(start, end);
+    if (self is ListSlice<E>) return self.slice(start, end);
     return ListSlice<E>(this, start, end);
   }
 
@@ -404,7 +404,7 @@ class ListSlice<E> extends ListBase<E> {
   /// ```
   ListSlice<E> slice(int start, [int? end]) {
     end = RangeError.checkValidRange(start, end, length);
-    return ListSlice._(_initialSize, source, start + start, end - start);
+    return ListSlice._(_initialSize, source, this.start + start, end - start);
   }
 
   @override

test/extensions_test.dart

@@ -1207,6 +1207,19 @@ void main() {
       test('refuses length 0', () {
         expect(() => iterable([1]).slices(0), throwsRangeError);
       });
+      test('regression #286, bug in slice', () {
+        var l1 = <int>[1, 2, 3, 4, 5, 6];
+        List<int> l2 = l1.slice(1, 5); // (2..5)
+        // This call would stack-overflow due to a lacking type promotion
+        // which caused the extension method to keep calling itself,
+        // instead of switching to the instance method on `ListSlice`.
+        //
+        // If successful, it would use the `2` argument as offset, instead
+        // of the `1` offset from the `slice` call above.
+        var l3 = l2.slice(2, 4); // (4..5)
+        expect(l3, [4, 5]);
+        expect(l3.toList(), [4, 5]);
+      });
     });
   });