fix(file_server): don't crash on "%" pathname (denoland#3953)

std/http/file_server.ts

@@ -301,9 +301,15 @@ function html(strings: TemplateStringsArray, ...values: unknown[]): string {
 listenAndServe(
   addr,
   async (req): Promise<void> => {
-    const normalizedUrl = posix.normalize(req.url);
-    const decodedUrl = decodeURIComponent(normalizedUrl);
-    const fsPath = posix.join(target, decodedUrl);
+    let normalizedUrl = posix.normalize(req.url);
+    try {
+      normalizedUrl = decodeURIComponent(normalizedUrl);
+    } catch (e) {
+      if (!(e instanceof URIError)) {
+        throw e;
+      }
+    }
+    const fsPath = posix.join(target, normalizedUrl);
 
     let response: Response;
     try {

std/http/file_server_test.ts

@@ -83,12 +83,15 @@ test(async function serveFallback(): Promise<void> {
   }
 });
 
-test(async function serveFallback(): Promise<void> {
+test(async function serveWithUnorthodoxFilename(): Promise<void> {
   await startFileServer();
   try {
-    const res = await fetch(
-      "http://localhost:4500/http/testdata/test%20file.txt"
-    );
+    let res = await fetch("http://localhost:4500/http/testdata/%");
+    assert(res.headers.has("access-control-allow-origin"));
+    assert(res.headers.has("access-control-allow-headers"));
+    assertEquals(res.status, 200);
+
+    res = await fetch("http://localhost:4500/http/testdata/test%20file.txt");
     assert(res.headers.has("access-control-allow-origin"));
     assert(res.headers.has("access-control-allow-headers"));
     assertEquals(res.status, 200);