Separate the cronet callbacks from the `send` method #789

Job	Run time
analyze_and_format; Dart 2.19.0; PKG: pkgs/http_client_conformance_tests; `dart analyze --fatal-i...	42s
analyze_and_format; Dart 3.0.0; PKG: pkgs/http; `dart analyze --fatal-infos`	24s
mono_repo self validate	25s
analyze_and_format; Dart dev; PKGS: pkgs/http, pkgs/http_client_conformance_tests; `dart analyze ...	31s
analyze_and_format; Dart dev; PKGS: pkgs/http, pkgs/http_client_conformance_tests; `dart format -...	31s
unit_test; Dart dev; PKG: pkgs/http; `dart run --define=no_default_http_client=true test/no_defau...	23s
unit_test; Dart 3.0.0; PKG: pkgs/http; `dart test --platform vm`	43s
unit_test; Dart 3.0.0; PKG: pkgs/http; `dart run --define=no_default_http_client=true test/no_def...	22s
unit_test; Dart 3.0.0; PKG: pkgs/http; `dart test --platform chrome`	2m 20s
unit_test; Dart dev; PKG: pkgs/http; `dart test --platform chrome`	2m 50s
unit_test; Dart dev; PKG: pkgs/http; `dart test --platform vm`	40s
	9m 51s

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