Removes the disable-upgrade feature from Polymer 2.0.

Due to #4550, the feature has a flaw for native ES6 classes and would be better implemented as either a mixin or patch to `customElements.define`.

lib/mixins/element-mixin.html

@@ -124,8 +124,6 @@
 
     let caseMap = Polymer.CaseMap;
 
-    const DISABLED = 'disable-upgrade';
-
     /**
      * Returns the `properties` object specifically on `klass`. Use for:
      * (1) super chain mixes togther to make `propertiesForClass` which is
@@ -453,8 +451,6 @@
       proto._bindTemplate(template);
     }
 
-    function flushPropertiesStub() {}
-
     /**
      * @polymerMixinClass
      * @unrestricted
@@ -472,7 +468,7 @@
        */
       static get observedAttributes() {
         if (!this.hasOwnProperty(JSCompiler_renameProperty('__observedAttributes', this))) {
-          let list = [DISABLED];
+          let list = [];
           let properties = propertiesForClass(this);
           for (let prop in properties) {
             list.push(Polymer.CaseMap.camelToDashCase(prop));
@@ -720,19 +716,7 @@
        * @override
        */
       attributeChangedCallback(name, old, value) {
-        // process `disable-upgrade` specially:
-        // First we see `disable-upgrade` added and disable `_flushProperties`,
-        // then when it's removed, restore regular flushing and flush.
-        // This should only be allowed before "readying".
-        if (name === DISABLED && !this.__dataInitialized) {
-          if (value !== null) {
-            this.__flushProperties = this._flushProperties;
-            this._flushProperties = flushPropertiesStub;
-          } else {
-            this._flushProperties = this.__flushProperties;
-            this._flushProperties();
-          }
-        } else if (old !== value) {
+        if (old !== value) {
           let property = caseMap.dashToCamelCase(name);
           let type = propertiesForClass(this.constructor)[property].type;
           if (!this._hasReadOnlyEffect(property)) {

lib/mixins/template-stamp.html

@@ -118,11 +118,6 @@
      */
     class TemplateStamp extends superClass {
 
-      _initializeProperties() {
-        super._initializeProperties();
-        this.$ = null;
-      }
-
       /**
        * Scans a template to produce template metadata.
        *

test/runner.html

@@ -71,8 +71,7 @@
       'unit/logging.html',
       'unit/mixin-utils.html',
       'unit/mixin-behaviors.html',
-      'unit/render-status.html',
-      'unit/disable-upgrade.html'
+      'unit/render-status.html'
     ];
 
     // http://eddmann.com/posts/cartesian-product-in-javascript/