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

sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate

Index: sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate
diff --git a/sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate b/sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate
index dda5aeebbf810fd9a8cc147a17e7e093c91d4686..e989a5e0e9314e49c46baad8710f2873c6a34e43 100644
--- a/sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate
@@ -5,8 +5,25 @@
 part of html;
 
 /// @domName $DOMNAME
+@SupportedBrowser(SupportedBrowser.CHROME)
+@SupportedBrowser(SupportedBrowser.FIREFOX)
+@SupportedBrowser(SupportedBrowser.SAFARI)
+@Experimental()
 class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
 $!MEMBERS
+  /**
+   * Checks to see if the mutation observer API is supported on the current
+   * platform.
+   */
+  static bool get supported {
+$if DARTIUM
+    return true;
+$else
+    return JS('bool',
+        '!!(window.MutationObserver || window.WebKitMutationObserver)');
+$endif
+  }
+
   void observe(Node target,
                {Map options,
                 bool childList,