dart2js: fix dart2js crash when getRuntimeTypeArgument was not registered.

tests/language/function_type3_test.dart

+// Copyright (c) 2015, 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.
+
+import "package:expect/expect.dart";
+
+class A<T> {
+  @NoInlining()
+  A();
+
+  @NoInlining()
+  foo() => new B<T>();
+}
+
+class B<T> {
+  T bar() => null;
+}
+
+typedef F();
+typedef F2(x);
+
+// Dart2js realized that the generic type for A was not needed, but then used
+// it nevertheless when it constructed the closure.
+main() {
+  var f = new A<int>().foo().bar;
+  Expect.isTrue(f is F);
+  Expect.isFalse(f is F2);
+}