Don't load assets unnecessarily when precompiling executables.

sdk/lib/_internal/pub/test/get/switch_source_test.dart

Index: sdk/lib/_internal/pub/test/get/switch_source_test.dart
diff --git a/sdk/lib/_internal/pub/test/get/switch_source_test.dart b/sdk/lib/_internal/pub/test/get/switch_source_test.dart
index 978dee827b74451865b7d9c17fcc4ac98c47a341..4b8ad77777adc839e3ded960271583ad129a8f8e 100644
--- a/sdk/lib/_internal/pub/test/get/switch_source_test.dart
+++ b/sdk/lib/_internal/pub/test/get/switch_source_test.dart
@@ -10,7 +10,7 @@ import '../test_pub.dart';
 main() {
   initConfig();
   integration('re-gets a package if its source has changed', () {
-    servePackages([packageMap("foo", "1.2.3")]);
+    servePackages((builder) => builder.serve("foo", "1.2.3"));
 
     d.dir('foo', [
       d.libDir('foo', 'foo 0.0.1'),