test: Add a test harness for the built card (#2653)

Mounts the card from `dist/` the way Home Assistant does, by URL with a
HACS tag, and asserts the entry stays a facade, that no chunk imports
it, that the browser is served the file unmodified, and that the card
starts up and fetches a language chunk lazily.

Runs in Chromium, Firefox and WebKit, so a change of bundler or minifier
is checked against every engine.

scripts/dist-commands.js

@@ -0,0 +1,56 @@
+import { existsSync, readdirSync, readFileSync } from 'node:fs';
+import path from 'node:path';
+
+import { init, parse } from 'es-module-lexer';
+
+const DIST_DIRECTORY = 'dist';
+
+const listFiles = () =>
+  existsSync(DIST_DIRECTORY)
+    ? readdirSync(DIST_DIRECTORY, { withFileTypes: true })
+        .filter((entry) => entry.isFile())
+        .map((entry) => entry.name)
+    : [];
+
+/**
+ * What a file imports, split into static imports, which the browser fetches
+ * before the file runs, and dynamic ones, which it fetches only when reached.
+ */
+const getImports = async (source) => {
+  await init;
+  const [imports] = parse(source);
+
+  // `n` is the file named in the import, and is absent when that name is put
+  // together at runtime, which names no one file.
+  const named = imports.filter((entry) => entry.n);
+
+  // `d` says how it is imported: -1 static, -2 `import.meta`, otherwise the
+  // position of the `import(`.
+  return {
+    staticImports: named.filter((entry) => entry.d === -1).map((entry) => entry.n),
+    dynamicImports: named.filter((entry) => entry.d >= 0).map((entry) => entry.n),
+  };
+};
+
+/**
+ * Facts about the built output, gathered in Node and handed to a test running
+ * in the browser.
+ *
+ * The imports are parsed rather than searched for: the card holds its own
+ * filenames as plain strings, which a text search would mistake for imports.
+ *
+ * @type {Record<string, import('vitest/node').BrowserCommand<[], unknown>>}
+ */
+export const distCommands = {
+  listDistFiles: () => listFiles(),
+
+  getDistImportGraph: async () => {
+    const graph = {};
+    for (const file of listFiles().filter((name) => name.endsWith('.js'))) {
+      graph[file] = await getImports(
+        readFileSync(path.resolve(DIST_DIRECTORY, file), 'utf8'),
+      );
+    }
+    return graph;
+  },
+};