Add tests for generic methods in strong mode

tests/language_strong/generic_methods_generic_class_tearoff_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.
+
+library generic_methods_generic_class_tearoff_test;
+
+import "package:expect/expect.dart";
+
+class A<T> {
+  T fun(T t) => t;
+}
+
+typedef Int2Int = int Function(int);
+typedef String2String = String Function(String);
+typedef Object2Object = Object Function(Object);
+typedef GenericMethod = T Function<T>(T);
+
+main() {
+  A<int> x = new A<int>();
+  var f = x.fun;
+  A<String> y = new A<String>();
+  var g = y.fun;
+  A z = new A();

[eernst, 2017/03/09 14:55:02]

This one depends on instantiate-to-bound for a raw type, which is currently
dynamic (we discussed Object, but I'll assume we stick to dynamic).

In that case `h` will have reified type `dynamic Function(Object)`.

I think it would be useful to mention these reified types here (as a comment on
f, g, and h), such that it is easier to see the intentions behind each case
below, if the test breaks at some point.

[Dmitry Stefantsov, 2017/03/10 13:21:53]

Thank you! Done.

+  var h = z.fun;
+
+  Expect.isTrue(f is Int2Int);
+  Expect.isTrue(f is! String2String);
+  Expect.isTrue(f is! Object2Object);

[Leaf, 2017/03/09 05:51:19]

I think this one should be true.  The reified type of `f` is `Object -> int`
under the current proposal, if I recall correctly, which is a subtype of `Object
-> Object`. 

[karlklose, 2017/03/09 09:17:00]

Why is it not `int -> int`?

[karlklose, 2017/03/09 09:18:53]

Or do you mean that 'int -> int' is a subtype of 'Object -> Object'?

[floitsch, 2017/03/09 11:27:17]

Because of covariant generics.
We have to support:
A<Object> tt = x;
Object Function(Object) f = tt.fun;

We don't want to do a dynamic check every time a method is torn off, which means
that we will dynamically make the tear-off be `int Function(Object)`. The
function will dynamically make sure that the given argument has the correct
type.

[Leaf, 2017/03/09 19:17:26]

Is the writeup of that ready to share yet?

[eernst, 2017/03/10 13:13:51]

We did that already:
https://github.com/dart-lang/sdk/blob/master/docs/language/informal/covariant...

[Dmitry Stefantsov, 2017/03/10 13:21:53]

Thank you! That explains a lot. 

+  Expect.isTrue(f is! GenericMethod);
+
+  Expect.isTrue(g is! Int2Int);
+  Expect.isTrue(g is String2String);
+  Expect.isTrue(g is! Object2Object);

[Leaf, 2017/03/09 05:51:18]

As above.

[Dmitry Stefantsov, 2017/03/10 13:21:53]

Done.

+  Expect.isTrue(g is! GenericMethod);
+
+  Expect.isTrue(h is! Int2Int);
+  Expect.isTrue(h is! String2String);
+  Expect.isTrue(h is Object2Object);
+  Expect.isTrue(h is! GenericMethod);
+}