Some changes to improve precision of types in dart:html

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 45d337bbd96736ba74c5150c5010e1bb1c9903ff..3bca460622e00c3d6e665375c99f0271803f3fe7 100644
--- a/sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_MutationObserver.darttemplate
@@ -62,7 +62,7 @@ $if DARTIUM
   static _add(m, String key, value) { m[key] = value; }
   static _fixupList(list) => list;
 
-  _call(Node target, options) {
+  void _call(Node target, options) {
     _observe(target, options);
   }
 $endif
@@ -73,6 +73,6 @@ $if DART2JS
   static _fixupList(list) => list;  // TODO: Ensure is a JavaScript Array.
 
   // Call native function with no conversions.
-  _call(target, options) native 'observe';
+  void _call(target, options) native 'observe';
 $endif
 }