Reuse a single TreeWalker in nested templates (lit#825)

Creating `TreeWalker`s isn't very optimized, so reusing a single one in the nested templates can speed it up a bit.

https://jsbench.github.io/#d0363ded1753dec3e060e61e1794896b

src/lib/template-instance.ts

@@ -68,14 +68,14 @@ export class TemplateInstance {
     const parts = this.template.parts;
     let partIndex = 0;
     let nodeIndex = 0;
+    // Edge needs all 4 parameters present; IE11 needs 3rd parameter to be null
+    const walker = document.createTreeWalker(
+        document,
+        133 /* NodeFilter.SHOW_{ELEMENT|COMMENT|TEXT} */,
+        null,
+        false);
     const _prepareInstance = (fragment: DocumentFragment) => {
-      // Edge needs all 4 parameters present; IE11 needs 3rd parameter to be
-      // null
-      const walker = document.createTreeWalker(
-          fragment,
-          133 /* NodeFilter.SHOW_{ELEMENT|COMMENT|TEXT} */,
-          null,
-          false);
+      walker.currentNode = fragment;
       let node = walker.nextNode();
       // Loop through all the nodes and parts of a template
       while (partIndex < parts.length && node !== null) {
@@ -103,6 +103,7 @@ export class TemplateInstance {
           nodeIndex++;
           if (node.nodeName === 'TEMPLATE') {
             _prepareInstance((node as HTMLTemplateElement).content);
+            walker.currentNode = node;
           }
           node = walker.nextNode();
         }

src/lib/template.ts

@@ -49,15 +49,14 @@ export class Template {
     let index = -1;
     let partIndex = 0;
     const nodesToRemove: Node[] = [];
+    // Edge needs all 4 parameters present; IE11 needs 3rd parameter to be null
+    const walker = document.createTreeWalker(
+        document,
+        133 /* NodeFilter.SHOW_{ELEMENT|COMMENT|TEXT} */,
+        null,
+        false);
     const _prepareTemplate = (template: HTMLTemplateElement) => {
-      const content = template.content;
-      // Edge needs all 4 parameters present; IE11 needs 3rd parameter to be
-      // null
-      const walker = document.createTreeWalker(
-          content,
-          133 /* NodeFilter.SHOW_{ELEMENT|COMMENT|TEXT} */,
-          null,
-          false);
+      walker.currentNode = template.content;
       // Keeps track of the last index associated with a part. We try to delete
       // unnecessary nodes, but we never want to associate two different parts
       // to the same index. They must have a constant node between.
@@ -103,6 +102,7 @@ export class Template {
           }
           if ((node as Element).tagName === 'TEMPLATE') {
             _prepareTemplate(node as HTMLTemplateElement);
+            walker.currentNode = node;
           }
         } else if (node.nodeType === 3 /* Node.TEXT_NODE */) {
           const data = (node as Text).data!;