Simplify host stack, set __dataHost unconditionally, and make _regist…

…erHost patchable.

lib/mixins/property-effects.js

@@ -899,6 +899,8 @@ function setupBindings(inst, templateInfo) {
           addNotifyListener(node, inst, binding);
         }
       }
+      // This ensures all bound elements have a host set, regardless
+      // of whether they upgrade synchronous to creation
       node.__dataHost = inst;
     }
   }
@@ -1376,7 +1378,7 @@ export const PropertyEffects = dedupingMixin(superClass => {
      */
     _initializeProperties() {
       super._initializeProperties();
-      hostStack.registerHost(this);
+      this._registerHost();
       this.__dataClientsReady = false;
       this.__dataPendingClients = null;
       this.__dataToNotify = null;
@@ -1389,6 +1391,16 @@ export const PropertyEffects = dedupingMixin(superClass => {
       this.__dataClientsInitialized = false;
     }
 
+    _registerHost() {
+      if (hostStack.length) {
+        let host = hostStack[hostStack.length-1];
+        host._enqueueClient(this);
+        // This ensures even non-bound elements have a host set, as
+        // long as they upgrade synchronously
+        this.__dataHost = host;
+      }
+    }
+
     /**
      * Overrides `PropertyAccessors` implementation to provide a
      * more efficient implementation of initializing properties from
@@ -2769,9 +2781,9 @@ export const PropertyEffects = dedupingMixin(superClass => {
       templateInfo =  templateInfo || /** @type {!TemplateInfo} */(this._bindTemplate(template, true));
       // Ensures that created dom is `_enqueueClient`'d to this element so
       // that it can be flushed on next call to `_flushProperties`
-      hostStack.beginHosting(this);
+      hostStack.push(this);
       let dom = super._stampTemplate(template, templateInfo);
-      hostStack.endHosting(this);
+      hostStack.pop();
       // Add template-instance-specific data to instanced templateInfo
       templateInfo.nodeList = dom.nodeList;
       // Capture child nodes to allow unstamping of non-prototypical templates
@@ -3164,7 +3176,7 @@ export const PropertyEffects = dedupingMixin(superClass => {
 });
 
 /**
- * Helper api for enqueuing client dom created by a host element.
+ * Stack for enqueuing client dom created by a host element.
  *
  * By default elements are flushed via `_flushProperties` when
  * `connectedCallback` is called. Elements attach their client dom to
@@ -3186,39 +3198,4 @@ export const PropertyEffects = dedupingMixin(superClass => {
  *
  * @private
  */
-class HostStack {
-  constructor() {
-    this.stack = [];
-  }
-
-  /**
-   * @param {*} inst Instance to add to hostStack
-   * @return {void}
-   */
-  registerHost(inst) {
-    if (this.stack.length) {
-      let host = this.stack[this.stack.length-1];
-      host._enqueueClient(inst);
-    }
-  }
-
-  /**
-   * @param {*} inst Instance to begin hosting
-   * @return {void}
-   */
-  beginHosting(inst) {
-    this.stack.push(inst);
-  }
-
-  /**
-   * @param {*} inst Instance to end hosting
-   * @return {void}
-   */
-  endHosting(inst) {
-    let stackLen = this.stack.length;
-    if (stackLen && this.stack[stackLen-1] == inst) {
-      this.stack.pop();
-    }
-  }
-}
-const hostStack = new HostStack();
+const hostStack = [];