net/http: fix build errors on js/wasm target

The in-progress WASM port does not yet have sufficient automatic
testing performed against it, so these errors slipped through when
adding the new Fetch API backed http.Roundtripper.

Updates #25506

Author: johanbrandhorst

src/net/http/roundtrip_js.go

@@ -18,7 +18,7 @@ import (
 )
 
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
-func (*Transport) RoundTrip(req *Request) (*Response, error) {
+func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	if useFakeNetwork() {
 		return t.roundTrip(req)
 	}
@@ -134,6 +134,8 @@ func (*Transport) RoundTrip(req *Request) (*Response, error) {
 	}
 }
 
+var errClosed = errors.New("net/http: reader is closed")
+
 // useFakeNetwork is used to determine whether the request is made
 // by a test and should be made to use the fake in-memory network.
 func useFakeNetwork() bool {