Add testing only stubs to dart:_http, and make it available for use in
the standalone tests so that the tests can stop including platform
library code directly using "part" directives (which can cause
surprising breakage, since the same code gets included twice via
different uris).
TEST=Existing tests
Change-Id: I1b5a5061008ef36980bd21b46a9d0fd701286f66
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/262780
Reviewed-by: Kallen Tu <kallentu@google.com>
Reviewed-by: Lasse Nielsen <lrn@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Leaf Petersen
131da0516b