Fix getSetterType in presence of metadata

tests/lib_strong/mirrors/field_metadata_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:mirrors';
 import 'package:expect/expect.dart';
 
 class Bar {
   final String name;
 
   const Bar(this.name);
 }
 
 class Foo {
   @Bar('bar')
   int x = 40;
 
   @Bar('baz')
   final String y = 'hi';
+
+  @Bar('foo')
+  void set z(int val) {
+    x = val;
+  }
 }
 
 void main() {
   dynamic f = new Foo();
   Expect.throws(() {
     f.x = 'hello';
   });
   f.x += 2;
   Expect.equals(f.x, 42);
   Expect.equals(f.y, 'hi');
 
+  Expect.throws(() {
+    f.z = 'hello';
+  });
+  f.z = 0;
+  Expect.equals(f.x, 0);
+
   var members = reflect(f).type.declarations;
   var x = members[#x] as VariableMirror;
   var y = members[#y] as VariableMirror;
   Expect.equals(x.type.simpleName, #int);
   Expect.equals(y.type.simpleName, #String);
 }