Import //fetch from Web Platform Tests.

third_party/WebKit/LayoutTests/external/wpt/fetch/api/basic/stream-response.js

+if (this.document === undefined) {
+  importScripts("/resources/testharness.js");
+  importScripts("../resources/utils.js");
+}
+
+function streamBody(reader, test, count) {
+  return reader.read().then(function(data) {
+    if (!data.done && count < 2) {
+      count += 1;
+      return streamBody(reader, test, count);
+    } else {
+      test.step(function() {
+        assert_true(count >= 2, "Retrieve body progressively");
+      });
+    }
+  });
+}
+
+//simulate streaming:
+//count is large enough to let the UA deliver the body before it is completely retrieved
+promise_test(function(test) {
+  return fetch(RESOURCES_DIR + "trickle.py?ms=30&count=100").then(function(resp) {
+    var count = 0;
+    if (resp.body)
+      return streamBody(resp.body.getReader(), test, count);
+    else
+      test.step(function() {
+        assert_unreached( "Body does not exist in response");
+      });
+  });
+}, "Stream response's body");
+
+done();