Uses mojom module names as Dart's package: import URI

mojo/dart/embedder/BUILD.gn

Index: mojo/dart/embedder/BUILD.gn
diff --git a/mojo/dart/embedder/BUILD.gn b/mojo/dart/embedder/BUILD.gn
index eb67d92f30eec520475e6aff72abede56f2212a3..1a11c9a434f86879a09f828a57e5d530c237ee11 100644
--- a/mojo/dart/embedder/BUILD.gn
+++ b/mojo/dart/embedder/BUILD.gn
@@ -76,14 +76,17 @@ group("dart_embedder_packages") {
 
 dart_embedder_package("dart_embedder_package_application") {
   package = "mojo/public/interfaces/application"
+  destination = "mojo"
 }
 
 dart_embedder_package("dart_embedder_package_bindings") {
   package = "mojo/public/interfaces/bindings"
+  destination = "mojo"
 }
 
 dart_embedder_package("dart_embedder_package_network_service") {
   package = "mojo/services/network/public/interfaces"
+  destination = "mojo"
 }
 
 action_foreach("dart_embedder_package_sdk") {
@@ -317,21 +320,23 @@ action("generate_snapshot_bin") {
   url_mappings_script =
       rebase_path("//mojo/dart/embedder/tools/dart_embedder_url_mappings.py")
 
+  application_mojoms = rebase_path("//mojo/public/interfaces/application")
+  bindings_mojoms = rebase_path("//mojo/public/interfaces/bindings")
+  network_service_mojoms =
+      rebase_path("//mojo/services/network/public/interfaces")
+
   # This script generates --url_mapping command line arguments for all
   # generated mojom.dart bindings files. See HACKING.md for more information.
   embedder_mojom_url_mappings =
       exec_script(url_mappings_script,
                   [
-                    # Imports are relative to source tree root.
-                    rebase_path("//"),
-
                     # File system path containing target imports.
                     rebase_path("$root_gen_dir/dart_embedder_packages"),
 
                     # Embedder package directories.
-                    rebase_path("//mojo/public/interfaces/application"),
-                    rebase_path("//mojo/public/interfaces/bindings"),
-                    rebase_path("//mojo/services/network/public/interfaces"),
+                    "$application_mojoms,mojo",
+                    "$bindings_mojoms,mojo",
+                    "$network_service_mojoms,mojo",
                   ],
                   "list lines")
 
@@ -339,7 +344,7 @@ action("generate_snapshot_bin") {
            "--executable",
            rebase_path("$gen_snapshot_dir/gen_snapshot"),
            "--package_root",
-           rebase_path("$root_gen_dir"),
+           rebase_path("$root_gen_dir/dart-gen"),
            "--script",
            rebase_path("snapshot.dart"),
            "--vm_output_bin",