Adding annotation for partial browser support and example implementation for MutationObservers.

sdk/lib/html/html_common/metadata.dart

+// Copyright (c) 2012, 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.
+
+part of html_common;
+
+/**
+ * An annotation used to mark a feature as only being supported by a subset
+ * of the browsers that Dart supports by default.
+ *
+ * If an API is not annotated with [SupportedBrowser] then it is assumed to
+ * work on all browsers Dart supports.
+ */
+class SupportedBrowser {
+  static const String CHROME = "Chrome";
+  static const String FIREFOX = "Firefox";
+  static const String IE = "Internet Explorer";
+  static const String SAFARI = "Safari";
+
+  /// The name of the browser.
+  final String browserName;
+  /// The minimum version of the browser that supports the feature, or null
+  /// if supported on all versions.
+  final String minimumVersion;
+
+  const SupportedBrowser(this.browserName, [this.minimumVersion]);
+}
+
+
+/**
+ * An annotation used to mark an API as being experimental.
+ *
+ * An API is considered to be experimental if it is still going through the
+ * process of stabilizing and is subject to change or removal.
+ *
+ * See also:
+ *
+ * * [W3C recommendation](http://en.wikipedia.org/wiki/W3C_recommendation)
+ */
+class Experimental {
+  const Experimental();
+}