[Fetch] Object.prototype shouldn't interfere with fetch streams

Test that setting Object.prototype.start to a function that throws
doesn't interfere with fetch operations that create a stream. Similarly,
verify that setting throwing accessors for 'type', 'size', and
'highWaterMark' doesn't cause problems.

See whatwg/fetch#781 for background.

fetch/api/basic/stream-safe-creation.any.js

@@ -0,0 +1,33 @@
+// These tests verify that stream creation is not affected by changes to
+// Object.prototype.
+
+const creationCases = {
+  fetch: async () => fetch(location.href),
+  request: () => new Request(location.href),
+  response: () => new Response('hi'),
+  consumeEmpty: () => new Response().text(),
+  consumeNonEmpty: () => new Response(new Uint8Array([64])).text()
+};
+
+for (creationCase of Object.keys(creationCases)) {
+  promise_test(async t => {
+    Object.prototype.start = () => {
+      throw Error('Object.prototype.start was called');
+    };
+    t.add_cleanup(() => delete Object.prototype.start);
+    await creationCases[creationCase];
+  }, `throwing Object.prototype.start() should not affect stream creation by ` +
+     `'${creationCase}'`);
+
+  for (accessorName of ['type', 'size', 'highWaterMark']) {
+    promise_test(async t => {
+      Object.defineProperty(Object.prototype, accessorName, {
+        get() { throw Error(`Object.prototype.${accessorName} was accessed`); },
+        configurable: true
+      });
+      t.add_cleanup(() => delete Object.prototype[accessorName]);
+      await creationCases[creationCase];
+    }, `throwing Object.prototype.${accessorName} accessor should not affect ` +
+       `stream creation by '${creationCase}'`);
+  }
+}