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

mojo/dart/embedder/test/BUILD.gn

Index: mojo/dart/embedder/test/BUILD.gn
diff --git a/mojo/dart/embedder/test/BUILD.gn b/mojo/dart/embedder/test/BUILD.gn
index e7c91356925ff438c427490535dbc37d9f214db8..96320c9836859ff954fd35dc1b7a7db03746ce43 100644
--- a/mojo/dart/embedder/test/BUILD.gn
+++ b/mojo/dart/embedder/test/BUILD.gn
@@ -31,7 +31,7 @@ copy("copy_dart_mojo_sdk") {
     "//mojo/public/dart/src/types.dart",
   ]
   outputs = [
-    "{{source_gen_dir}}/{{source_file_part}}",
+    "$root_gen_dir/dart-gen/{{source_root_relative_dir}}/{{source_file_part}}",
   ]
 }
 
@@ -77,7 +77,7 @@ copy("expect") {
     "//mojo/dart/testing/expect.dart",
   ]
   outputs = [
-    "{{source_gen_dir}}/{{source_file_part}}",
+    "$root_gen_dir/dart-gen/{{source_root_relative_dir}}/{{source_file_part}}",
   ]
 }
 
@@ -86,7 +86,7 @@ copy("validation_test_input_parser") {
     "//mojo/dart/testing/validation_test_input_parser.dart",
   ]
   outputs = [
-    "{{source_gen_dir}}/{{source_file_part}}",
+    "$root_gen_dir/dart-gen/{{source_root_relative_dir}}/{{source_file_part}}",
   ]
 }
 
@@ -95,7 +95,7 @@ copy("async_helper") {
     "//mojo/dart/testing/async_helper.dart",
   ]
   outputs = [
-    "{{source_gen_dir}}/{{source_file_part}}",
+    "$root_gen_dir/dart-gen/{{source_root_relative_dir}}/{{source_file_part}}",
   ]
 }