Properly handle unfinalized type parameters of generic mixins (fixes #26230).

tests/language/regress_26230_test.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class _RenderTabBar extends RenderBox with
+    ContainerRenderObjectMixin<RenderBox, _TabBarParentData>,
+    RenderBoxContainerDefaultsMixin<RenderBox, _TabBarParentData> { }
+class RenderObject { }
+class RenderSector extends RenderObject { }
+class RenderBox extends RenderObject { }
+class ParentData { }
+class BoxParentData extends ParentData { }
+class SectorParentData extends ParentData { }
+class ContainerParentDataMixin<ChildType extends RenderObject> { }
+class ContainerRenderObjectMixin<ChildType extends RenderObject, ParentDataType extends ContainerParentDataMixin<ChildType>> { }
+class SectorChildListParentData extends SectorParentData with ContainerParentDataMixin<RenderSector> { }
+class RenderDecoratedSector extends RenderSector { }
+class RenderSectorWithChildren extends RenderDecoratedSector with ContainerRenderObjectMixin<RenderSector, SectorChildListParentData> { }
+class ContainerBoxParentDataMixin<ChildType extends RenderObject> extends BoxParentData with ContainerParentDataMixin<ChildType> { }
+class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, ParentDataType extends ContainerBoxParentDataMixin<ChildType>> implements ContainerRenderObjectMixin<ChildType, ParentDataType> { }
+class FlexParentData extends ContainerBoxParentDataMixin<RenderBox> { }
+class _TabBarParentData extends ContainerBoxParentDataMixin<RenderBox> { }
+
+main() {
+  new _RenderTabBar();
+}
+