Merge pull request #21321 from johanrd/perf/pr-pathexpression-original-getter

Perf: avoid array spread in PathExpression.original getter

Author: NullVoxPopuli

packages/@glimmer/syntax/lib/v1/legacy-interop.ts

@@ -48,7 +48,8 @@ export function buildLegacyPath({ head, tail, loc }: PathExpressionParams): ASTv
     head,
     tail,
     get original() {
-      return [this.head.original, ...this.tail].join('.');
+      const head = this.head.original;
+      return this.tail.length === 0 ? head : `${head}.${this.tail.join('.')}`;
     },
     set original(value: string) {
       let [head, ...tail] = asPresentArray(value.split('.'));