feat(http): add basic http service

This implementation only works in JavaScript, while the Observable transpilation
story gets worked out. Right now, the service just makes a simple request,
and returns an Observable of Response.

Additional functionality will be captured in separate issues.

Fixes #2028

scripts/ci/test_e2e_dart.sh

@@ -16,6 +16,7 @@ function killServer () {
 serverPid=$!
 
 ./node_modules/.bin/gulp build.css.material&
+./node_modules/.bin/gulp build.http.example&
 
 trap killServer EXIT
 

scripts/ci/test_e2e_js.sh

@@ -16,6 +16,7 @@ function killServer () {
 serverPid=$!
 
 ./node_modules/.bin/gulp build.css.material&
+./node_modules/.bin/gulp build.http.example&
 
 trap killServer EXIT