fix wrong order to isAssignableTo for override params

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 '../../../reflective_tests.dart';
 import 'strong_test_helper.dart';
 
 void main() {
@@ skipping 2991 matching lines @@
   void test_optionalParams() {
     // Regression test for https://github.com/dart-lang/sdk/issues/26155
     checkFile(r'''
 void takesF(void f(int x)) {
   takesF(/*info:INFERRED_TYPE_CLOSURE,info:INFERRED_TYPE_CLOSURE*/([x]) { bool z = x.isEven; });
   takesF(/*info:INFERRED_TYPE_CLOSURE*/(y) { bool z = y.isEven; });
 }
     ''');
   }
 
+  void test_overrideNarrowsType() {
+    addFile(r'''
+class A {}
+class B extends A {}
+
+abstract class C {
+  m(A a);
+  n(B b);
+}
+abstract class D extends C {
+  /*error:INVALID_METHOD_OVERRIDE*/m(/*error:INVALID_METHOD_OVERRIDE_NORMAL_PARAM_TYPE*/B b);

[Jennifer Messerly, 2016/08/10 23:50:48]

we should probably let error_verifier's messages replace ours. it's a bit
confusing right now we do the checking in two places. e.g. for @covariant
overrides I will have to fix both.

+  n(A a);
+}
+    ''');
+    check(implicitCasts: false);

[Jennifer Messerly, 2016/08/10 23:50:48]

This bug wasn't visible until we turn off implicit casts. The reason is we have
our own override checking in checker.dart for strong mode, and isAssignableTo is
highly permissive even in strong mode. So we just didn't see this problem. Same
deal in Dart 1 -- the order (almost) didn't matter.

+  }
+
   void test_privateOverride() {
     addFile(
         '''
 import 'main.dart' as main;
 
 class Base {
   var f1;
   var _f2;
   var _f3;
   get _f4 => null;
@@ skipping 708 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();
 }
 ''');
   }
 }