Add test for issues with cascades.

tests/language/cascade_2_test.dart

+// Copyright (c) 2012, 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.
+
+// Test cascades, issue 7665.
+
+main() {
+  var a = new Element(null);
+  Expect.equals(1, a.path0.length);
+  Expect.equals(a, a.path0[0]);
+
+  Expect.equals(1, a.path1.length);  // 2 instead of 1
+  Expect.equals(a, a.path1[0]);
+
+  Expect.equals(1, a.path2.length);  // NPE.
+
+  var b = new Element(a);
+  Expect.equals(2, b.path0.length);
+  Expect.equals(a, b.path0[0]);
+  Expect.equals(b, b.path0[1]);
+
+  Expect.equals(2, b.path1.length);  // 3 instead of 2.
+  Expect.equals(a, b.path1[0]);
+  Expect.equals(b, b.path1[1]);
+
+  Expect.equals(2, b.path2.length);  // NPE.
+}
+
+
+class Element {
+  final Element parent;
+
+  Element(this.parent);
+
+  List<Element> get path0 {
+    if (parent == null) {
+      return <Element>[this];
+    } else {
+      return parent.path0..add(this);
+    }
+  }
+
+  List<Element> get path1 {
+    return (parent == null) ? <Element>[this] : parent.path1..add(this);
+  }
+
+  List<Element> get path2 {
+    return (parent == null) ? <Element>[this] : (parent.path2..add(this));
+  }
+}