Issue 29186. Make several top-level variables explicitly 'dynamic'.

tests/lib_strong/html/input_element_test.dart

 import 'dart:html';
 
 import 'package:expect/minitest.dart';
 
 void check(InputElement element, String type, [bool supported = true]) {
   expect(element is InputElement, true);
   if (supported) {
     expect(element.type, type);
   } else {
     expect(element.type, 'text');
@@ skipping 171 matching lines @@
       expect(e.value, '');
     });
     test('valueSetNullProxy', () {
       final e = new TextInputElement();
       e.value = _undefined;
       expect(e.value, '');
     });
   });
 }
 
-var _undefined = (() => new List(5)[0])();
+dynamic _undefined = (() => new List(5)[0])();