Add shim.js w/narrow_state global.

static/js/shim.js

@@ -0,0 +1,10 @@
+/*
+
+This module has shims to help us break circular dependencies.
+We eventually want to to move the actual implementations into
+new modules.  When we do this, you may need to fix node tests
+that still refer to the old name.
+*/
+
+var narrow_state = {}; // global, should be made into module
+narrow_state.set_compose_defaults = narrow.set_compose_defaults;