Package js_shared:

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.