Undo fix on IE10 where the custom elements polyfill's mixin strategy makes this unfeasible.

src/lib/settings.html

@@ -38,6 +38,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       var useNativeCustomElements = (!window.CustomElements ||
         window.CustomElements.useNative);
 
+      var usePolyfillProto = !useNativeCustomElements && !Object.__proto__;
+
       return {
         wantShadow: wantShadow,
         hasShadow: hasShadow,
@@ -45,7 +47,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         useShadow: useShadow,
         useNativeShadow: useShadow && nativeShadow,
         useNativeImports: useNativeImports,
-        useNativeCustomElements: useNativeCustomElements
+        useNativeCustomElements: useNativeCustomElements,
+        usePolyfillProto: usePolyfillProto
       };
     })()
   };

src/standard/configure.html

@@ -9,7 +9,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 -->
 
 <script>
-
+(function() {
   /*
     Process inputs efficiently via a configure lifecycle callback.
     Configure is called top-down, host before local dom. Users should
@@ -41,6 +41,8 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
   directly (on-foo-changed).
   */
 
+  var usePolyfillProto = Polymer.Settings.usePolyfillProto;
+
   Polymer.Base._addFeature({
 
     // storage for configuration
@@ -118,7 +120,7 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
         // to override default values. This allows late upgrade + an early set
         // to not b0rk accessors on the prototype.
         // Perf testing has shown `hasOwnProperty` to be ok here.
-        if (this.hasOwnProperty(i)) {
+        if (!usePolyfillProto && this.hasOwnProperty(i)) {
           config[i] = this[i];
           delete this[i];
         } else if (c.value !== undefined) {
@@ -223,4 +225,5 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 
   });
 
+})();
 </script>

test/unit/configure.html

@@ -104,6 +104,10 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     });
 
     test('pre-register property assignment does not break getters and setters', function() {
+      // don't test if __proto__ is polyfilled (IE10); cannot be fixed in this case.
+      if (Polymer.Settings.usePolyfillProto) {
+        return;
+      }
       var x = document.createElement('x-late-register');
       document.body.appendChild(x);
       // set property