fix #28630, instance method tear-offs are treated as strict arrows

pkg/analyzer/test/src/task/strong/checker_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.
 
 library analyzer.test.src.task.strong.checker_test;
 
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'strong_test_helper.dart';
 
@@ skipping 1656 matching lines @@
   C c = new C();
   {
     Function2<B, A> f;
     f = c.top;
     f = c.left;
     f = c.right;
     f = c.bot;
   }
   {
     Function2<B, B> f;
-    f = /*warning:DOWN_CAST_COMPOSITE*/c.top;
+    f = /*error:INVALID_CAST_METHOD*/c.top;

[Leaf, 2017/02/03 20:28:41]

This isn't right.    This error is only for casts that are guaranteed to fail. 
Here,  c could be a subtype of C which tightens the return type so that the cast
will succeed.

[Jennifer Messerly, 2017/02/03 21:02:50]

yeah, that's why I'd split out like that initially. Happy to keep the
distinction, though I may just inline that "hasKnownFunction" method so we don't
hit the problem again of using it inappropriately.

Eventually we want to remove implicit casts though, right? at which point it
will become an error?

     f = c.left;
     f = /*error:INVALID_ASSIGNMENT*/c.right;
     f = c.bot;
   }
   {
     Function2<A, A> f;
-    f = /*warning:DOWN_CAST_COMPOSITE*/c.top;
+    f = /*error:INVALID_CAST_METHOD*/c.top;
     f = /*error:INVALID_ASSIGNMENT*/c.left;
     f = c.right;
     f = c.bot;
   }
   {
     Function2<A, B> f;
-    f = /*warning:DOWN_CAST_COMPOSITE*/c.top;
-    f = /*warning:DOWN_CAST_COMPOSITE*/c.left;
-    f = /*warning:DOWN_CAST_COMPOSITE*/c.right;
+    f = /*error:INVALID_CAST_METHOD*/c.top;
+    f = /*error:INVALID_CAST_METHOD*/c.left;
+    f = /*error:INVALID_CAST_METHOD*/c.right;
     f = c.bot;
   }
 }
 ''');
   }
 
   void test_functionTypingAndSubtyping_intAndObject() {
     checkFile('''
 typedef Object Top(int x);      // Top of the lattice
 typedef int Left(int x);        // Left branch
@@ skipping 1967 matching lines @@
 
   void test_superConstructor() {
     checkFile('''
 class A { A(A x) {} }
 class B extends A {
   B() : super(/*error:ARGUMENT_TYPE_NOT_ASSIGNABLE*/3);
 }
 ''');
   }
 
+  void test_tearOffTreatedConsistentlyAsStrictArrow() {
+    checkFile(r'''
+void foo(void f(String x)) {}
+
+class A {
+  Null bar1(dynamic x) => null;
+  void bar2(dynamic x) => null;
+  Null bar3(String x) => null;
+  void test() {
+    foo(bar1);
+    foo(bar2);
+    foo(bar3);
+  }
+}
+
+
+Null baz1(dynamic x) => null;
+void baz2(dynamic x) => null;
+Null baz3(String x) => null;
+void test() {
+  foo(baz1);
+  foo(baz2);
+  foo(baz3);
+}
+    ''');
+  }
+
   void test_ternaryOperator() {
     checkFile('''
 abstract class Comparable<T> {
   int compareTo(T other);
   static int compare(Comparable a, Comparable b) => a.compareTo(b);
 }
 typedef int Comparator<T>(T a, T b);
 
 typedef bool _Predicate<T>(T value);
 
@@ skipping 397 matching lines @@
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
   Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }