Support `void` as generic argument.

tests/language/void_type4_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';
+
+use<T>(T x) {}
+
+class A<T> {
+  T x;
+  Object y;
+  int z;
+
+  T foo() => null;
+  void bar() {}
+  void gee(T x) {}
+
+  f(A<Object> a) {}
+  g(A<void> a) {}
+  h(A<T> a) {}
+}
+
+class B implements A<Object> {
+  void   /// 00: static type warning
+  /*     /// 00: continued
+  var
+  */     /// 00: continued
+  x;
+
+  void   /// 00b: static type warning
+  /*     /// 00b: continued
+  var
+  */     /// 00b: continued
+  y;
+
+  void   /// 00c: static type warning
+  /*     /// 00c: continued
+  int
+  */     /// 00c: continued
+  z;
+
+  // Overriding an Object function with a void function is an error.
+  void  /// 01: static type warning
+  foo() => null;
+
+  int bar() => 499;
+  void gee(void x) {}
+  f(A<void> a) {}
+  g(A<void> a) {}
+  h(A<void> a) {}
+}
+
+class C implements A<void> {
+  void x;
+  Object y;
+  int z;
+
+  void foo() {}
+  void bar() {}
+  void gee(void x) {
+    use(x);  /// 03: static type warning
+  }
+
+  f(C c) {}
+  g(C c) {}
+  h(C c) {}
+}
+
+class D implements A<void> {
+  Object x;  /// 04: static type warning
+  Object y;
+  int z;
+
+  Object foo() => null;
+  void bar() {}
+  void gee(
+    Object  /// 05: static type warning
+    x) {}
+
+  f(A<Object> a) {}
+  g(
+    A<Object>  /// 06: static type warning
+    a) {}
+  h(
+    A<Object>  /// 07: static type warning
+    a) {}
+}
+
+void instantiateClasses() {
+  var a = new A<void>();
+  var b = new B();
+  var c = new C();
+  var d = new D();
+
+  a.foo();
+  b.foo();
+  c.foo();
+  d.foo();
+  a.bar();
+  b.bar();
+  c.bar();
+  d.bar();
+  a.gee(499);
+  b.gee(499);
+  c.gee(499);
+  d.gee(499);
+}
+
+void testAssignments() {
+  A<void> a1 = new A<Object>();
+  A<Object> a2 = new A<void>();
+  A a3 = new A<void>();
+  A<dynamic> a4 = new A<void>();
+  dynamic a5 = new A<void>();
+}
+
+main() {
+  instantiateClasses();
+  testAssignments();
+}