ReadableStream.cancel's parameter should be optional.

LayoutTests/http/tests/fetch/script-tests/stream-reader.js

 if (self.importScripts) {
   importScripts('/fetch/resources/fetch-test-helpers.js');
 }
 
 function read_until_end(reader) {
   var chunks = [];
   function consume() {
     return reader.read().then(function(r) {
         if (r.done) {
           return chunks;
@@ skipping 84 matching lines @@
         return read_until_end(original.clone().body.getReader());
       }).then(function(chunks) {
         for (var chunk of chunks) {
           read += chunk.byteLength;
         }
         // Make sure that we received all data in total.
         assert_equals(read, 190);
       });
   }, 'Clone after reading partially');
 
+sequential_promise_test(function(t) {
+    var read = 0;

[tyoshino (SeeGerritForStatus), 2015/03/23 03:57:21]

remove

[yhirano, 2015/03/23 05:47:31]

Reverted

+    var original;
+    return fetch('/fetch/resources/progressive.php').then(function(res) {
+        res.body.cancel();
+        return res.text();
+      }).then(function(text) {
+        assert_equals(text, '');
+      });
+  }, 'Cancelling stream stops downloading.');
+
 sequential_promise_test_done();
 done();