Remove Collection, Collections and clean up List/Set/Queue implementations of retain/remove.

tools/dom/templates/html/impl/impl_Storage.darttemplate

Index: tools/dom/templates/html/impl/impl_Storage.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Storage.darttemplate b/tools/dom/templates/html/impl/impl_Storage.darttemplate
index e4d05237195e1361a3515192bea9f6346b9e5963..d043abef795a1360053b864c31dff4a285aac2f4 100644
--- a/tools/dom/templates/html/impl/impl_Storage.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Storage.darttemplate
@@ -63,13 +63,13 @@ $(ANNOTATIONS)class $CLASSNAME$EXTENDS implements Map<String, String>
     }
   }
 
-  Collection<String> get keys {
+  Iterable<String> get keys {
     final keys = [];
     forEach((k, v) => keys.add(k));
     return keys;
   }
 
-  Collection<String> get values {
+  Iterable<String> get values {
     final values = [];
     forEach((k, v) => values.add(v));
     return values;