fix #29530, field metadata generation

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';
+}
+
+void main() {
+  dynamic f = new Foo();
+  Expect.throws(() {
+    f.x = 'hello';
+  });
+  f.x += 2;
+  Expect.equals(f.x, 42);
+  Expect.equals(f.y, 'hi');
+
+  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);
+}