feat(etc/fetch): Support WebAssembly.instantiateStreaming for file fetches (#12901)

Fetching of local files, added in #12545, returns a response with no
headers, including the `Content-Type` header. This currently makes it
not work with the WebAssembly streaming APIs, which require the response
to have a content type of `application/wasm`.

Since the only way to obtain a `Response` object with a non-empty `url`
field is via `fetch()`, this change changes the content type requirement
to only apply to responses whose url has the `file:` scheme.

cli/tests/unit/wasm_test.ts

@@ -76,6 +76,20 @@ Deno.test(async function wasmInstantiateStreaming() {
   assertEquals(add(1, 3), 4);
 });
 
+Deno.test(
+  { permissions: { read: true } },
+  async function wasmFileStreaming() {
+    const url = new URL("../testdata/unreachable.wasm", import.meta.url);
+    assert(url.href.startsWith("file://"));
+
+    const { module } = await WebAssembly.instantiateStreaming(fetch(url));
+    assertEquals(WebAssembly.Module.exports(module), [{
+      name: "unreachable",
+      kind: "function",
+    }]);
+  },
+);
+
 Deno.test(
   { permissions: { net: true } },
   async function wasmStreamingNonTrivial() {

ext/fetch/26_fetch.js

@@ -37,6 +37,7 @@
     PromisePrototypeThen,
     PromisePrototypeCatch,
     String,
+    StringPrototypeStartsWith,
     StringPrototypeToLowerCase,
     TypedArrayPrototypeSubarray,
     TypeError,
@@ -498,6 +499,9 @@
         // The spec is ambiguous here, see
         // https://github.com/WebAssembly/spec/issues/1138. The WPT tests
         // expect the raw value of the Content-Type attribute lowercased.
+        // We ignore this for file:// because file fetches don't have a
+        // Content-Type.
+        if (!StringPrototypeStartsWith(res.url, "file://")) {
           const contentType = res.headers.get("Content-Type");
           if (
             typeof contentType !== "string" ||
@@ -505,6 +509,7 @@
           ) {
             throw new TypeError("Invalid WebAssembly content type.");
           }
+        }
 
         // 2.5.
         if (!res.ok) {