fix #27256, track type bounds for generic functions

tests/language_strong/generic_function_bounds_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.
+
+import 'dart:math';
+import 'package:expect/expect.dart';
+
+void testInstantiateToBounds() {
+  f<T extends num, U extends T>() => [T, U];
+  g<T extends List<U>, U extends int>() => [T, U];
+  h<T extends num, U extends T>(T x, U y) => h.runtimeType.toString();
+
+  Expect.listEquals((f as dynamic)(), [num, num]);
+  Expect.equals((g as dynamic)().join('|'), 'List<int>|int');
+  Expect.equals((h as dynamic)(null, null),
+      '<T extends num, U extends T>(T, U) -> String');
+}
+
+void testChecksBound() {
+  f<T extends num>(T x) => x;
+  Expect.equals((f as dynamic)(42), 42);
+  Expect.throws(() => (f as dynamic)('42'));
+
+  g<T extends U, U extends num>(T x, U y) => x;
+  Expect.equals((g as dynamic)(42.0, 100), 42.0);
+  Expect.throws(() => (g as dynamic)('hi', 100));
+}
+
+typedef G<U> = T Function<T extends U>(T x);
+
+void testSubtype() {
+  f<T extends num>(T x) => x + 2;
+
+  dynamic d = f;
+  Expect.equals(d(40.0), 42.0);
+  Expect.equals((f as G<int>)(40), 42);
+  Expect.equals((d as G<int>)(40), 42);
+  Expect.equals((f as G<double>)(40.0), 42.0);
+  Expect.equals((d as G<double>)(40.0), 42.0);
+
+  d as G<Null>;
+  Expect.throws(() => d as G);
+  Expect.throws(() => d as G<Object>);
+  Expect.throws(() => d as G<String>);
+}
+
+void testToString() {
+  // TODO(jmesserly): I don't think the cast on `y` should be required.
+  num f<T extends num, U extends T>(T x, U y) => min(x, y as num);
+  num g<T, U>(T x, U y) => max(x as num, y as num);
+  String h<T, U>(T x, U y) => h.runtimeType.toString();
+  Expect.equals(f.runtimeType.toString(), '<T extends num, U extends T>(T, U) -> num');
+  Expect.equals(g.runtimeType.toString(), '<T, U>(T, U) -> num');
+  Expect.equals(h(42, 123.0), '<T, U>(T, U) -> String');
+}
+
+main() {
+  testInstantiateToBounds();
+  testToString();
+  testChecksBound();
+  testSubtype();
+}