TODO: docs maybe??

The summary marker does not appear in the AOM, so setting its alt text
is fruitless.

cmd/pkgserver/ui/static/test.ts

@@ -281,10 +281,8 @@ export class StreamReporter implements Reporter {
     }
 }
 
-function assertGetElementById(id: string): HTMLElement {
-    let elem = document.getElementById(id);
-    if (elem == null) throw new ReferenceError(`element with ID '${id}' missing from DOM`);
-    return elem;
+function throwNotInDOMError(id: string): never {
+    throw new ReferenceError(`element with ID '${id}' missing from DOM`);
 }
 
 export class DOMReporter implements Reporter {
@@ -293,12 +291,16 @@ export class DOMReporter implements Reporter {
     update(path: string[], result: TestResult) {
         if (path.length === 0) throw new RangeError("path is empty");
         if (result.state === "skip") {
-            assertGetElementById("skip-counter-text").classList.remove("hidden");
+            const text = document.getElementById("skip-counter-text");
+            if (!text) throwNotInDOMError("skip-counter-text");
+            text.classList.remove("hidden");
         }
-        const counter = assertGetElementById(`${result.state}-counter`);
+        const counter = document.getElementById(`${result.state}-counter`);
+        if (!counter) throwNotInDOMError(`${result.state}-counter`);
         counter.innerText = (Number(counter.innerText) + 1).toString();
 
-        let parent = assertGetElementById("root");
+        let parent = document.getElementById("root");
+        if (!parent) throwNotInDOMError("root");
         for (const node of path) {
             let child: HTMLDetailsElement | null = null;
             let summary: HTMLElement | null = null;