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

pkg/analyzer/test/src/task/strong/checker_test.dart

Index: pkg/analyzer/test/src/task/strong/checker_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/checker_test.dart b/pkg/analyzer/test/src/task/strong/checker_test.dart
index 73fc16c5d48b058ac12fa345a192ac27da6a7e59..1d6d5db274190f3e404bd31803ea7b5f1d09edc7 100644
--- a/pkg/analyzer/test/src/task/strong/checker_test.dart
+++ b/pkg/analyzer/test/src/task/strong/checker_test.dart
@@ -1674,23 +1674,23 @@ void main() {
   }
   {
     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;
   }
 }
@@ -3678,6 +3678,33 @@ class B extends A {
 ''');
   }
 
+  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> {