Narrowing attribute maps to only accept strings.

tests/html/element_test.dart

 // Copyright (c) 2011, 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 ElementTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:async';
 import 'dart:html';
 import 'dart:svg' as svg;
@@ skipping 262 matching lines @@
       e.onClick.listen((event) {
         firedEvent = true;
       });
       expect(firedEvent, false);
       e.click();
       expect(firedEvent, true);
     });
   });
 
   group('attributes', () {
-      test('coercion', () {
-        final element = new Element.tag('div');
-        element.attributes['foo'] = 42;
-        element.attributes['bar'] = 3.1;
-        expect(element.attributes['foo'], '42');
-        expect(element.attributes['bar'], '3.1');
-      });
       test('manipulation', () {
         final element = new Element.html(
             '''<div class="foo" style="overflow: hidden" data-foo="bar"
                    data-foo2="bar2" dir="rtl">
                </div>''');
         final attributes = element.attributes;
         expect(attributes['class'], 'foo');
         expect(attributes['style'], startsWith('overflow: hidden'));
         expect(attributes['data-foo'], 'bar');
         expect(attributes['data-foo2'], 'bar2');
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     });
 
     test('sublist', () {
       var range = makeElList().sublist(1, 3);
       expect(range, isElementList);
       expect(range[0], isImageElement);
       expect(range[1], isInputElement);
     });
   });
 }