[1.x] Ensure template instance isn't cached on prototype. Fixes #5096

src/lib/template/templatizer.html

@@ -198,8 +198,13 @@
     },
 
     _customPrepAnnotations: function(archetype, template) {
-      archetype._template = template;
-      var c = template._content;
+      // Store a "clone" of the template on the archetype to ensure no
+      // references on this template are leaked onto the cached archetype
+      // Note we don't actually clone the template since it could be
+      // a type-extension; rather we create a new one and copy the
+      // `_content` reference holding the prototypical content over
+      var t = archetype._template = document.createElement('template');
+      var c = t._content = template._content;
       if (!c._notes) {
         var rootDataHost = archetype._rootDataHost;
         if (rootDataHost) {

test/unit/templatizer-elements.html

@@ -129,6 +129,8 @@
       });
       var parent = Polymer.dom(this).parentNode;
       Polymer.dom(parent).appendChild(this.instance.root);
+      // Ensure templatizer class does not leak reference to dataHost
+      assert.notOk(this.ctor.prototype._template.dataHost, 'templatizer class should not leak reference to dataHost');
     }
   });
 
@@ -196,6 +198,8 @@
       });
       var parent = Polymer.dom(this).parentNode;
       Polymer.dom(parent).appendChild(this.instance.root);
+      // Ensure templatizer class does not leak reference to dataHost
+      assert.notOk(this.ctor.prototype._template.dataHost, 'templatizer class should not leak reference to dataHost');
     }
   });