New test, checking allowed usages of void typed expressions

Change-Id: I9a3096bb3fb3deca178688029657cd71f8aedf7d
Reviewed-on: https://dart-review.googlesource.com/30840
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Erik Ernst <eernst@google.com>

tests/language_2/generalized_void_usage_test.dart

@@ -0,0 +1,37 @@
+// Copyright (c) 2017, 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.
+
+// Dart test for static checks on situations where expressions of type void
+// can be used. The point is simply that there are no compile-time errors.
+
+// We need expressions of type `void`; `void x` would do, but using
+// `void get x` will work with tools that do not yet handle `void x`.
+void get x => null;
+
+void use(dynamic x) {}
+
+main() {
+  // In an expressionStatement `e;`, e may have type void.
+  x;
+
+  // In the initialization and increment expressions of a for-loop,
+  // `for (e1; e2; e3) {..}`, `e1` and `e3` may have type void.
+  for (x;; x) {
+    break;
+  }
+
+  // In a typeCast `e as T`, `e` may have type void.
+  var y = x as Object;
+
+  // In a parenthesized expression `(e)`, `e` may have type void.
+  (x);
+
+  // In a return statement `return e;`, when the return type of the
+  // innermost enclosing function is the type void, e may have type void.
+  void f() => x;
+
+  void g() {
+    return x;
+  }
+}