@angular/http/testing used to publish a metadata structure which paralleled
the .d.ts structure. This causes ngc to write incorrect imports for this
bundle when compiling providers using MockBackend and other http testing
classes.
This change restructures the @angular/http/testing build a bit, modeling it
after @angular/platform-browser-animations, and produces a FESM structure
that has flat metadata.
Fixes#15521.
