This code is a compile time dependency of dart2js and DDC. It is imported as a
package: import by both compilers.
There is an exact copy in the SDK of the libraries in the
pkg/js_shared/lib/synced sub-directory. Those libraries are imported as
dart: imports by the dart2js and DDC runtime libraries.
Important: all code under
pkg/js_shared/lib/synced must be kept in sync with the runtime (in
sdk/lib/_internal/js_shared/lib/synced) at all times. The
test/in_sync_test.dart test verifies this.