Support `void` as generic argument.

tests/language/void_type5_test.dart

+// 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 type checks involving the void type.
+
+import 'package:expect/expect.dart';
+
+class A<T> {
+  T x;
+  foo(T x) {}
+  T bar(T f()) {
+    T tmp = f();
+    return tmp;
+  }
+
+  gee(T f()) {
+    x = bar(f);
+  }
+
+  Object xAsObject() => x;
+}
+
+void voidFun() => 499;
+int intFun() => 42;
+
+main() {
+  A<void> a = new A<void>();
+  a.foo(a.x);  /// 00: static type warning
+  a.bar(voidFun);
+  Expect.equals(null, a.xAsObject());
+  a.x = a.bar(voidFun);   /// 01: static type warning
+  a.gee(voidFun);  // But we can do it through `gee`.
+  Expect.equals(499, a.xAsObject());
+  a.gee(intFun);  // We can also pass in an int function.
+  Expect.equals(42, a.xAsObject());
+}