Don't load assets unnecessarily when precompiling executables.

sdk/lib/_internal/pub/test/snapshot/upgrades_snapshot_test.dart

Index: sdk/lib/_internal/pub/test/snapshot/upgrades_snapshot_test.dart
diff --git a/sdk/lib/_internal/pub/test/snapshot/upgrades_snapshot_test.dart b/sdk/lib/_internal/pub/test/snapshot/upgrades_snapshot_test.dart
index 35316821c1b951e0be2e0670fde9ef638a41f9f8..10aefb6bbb2024f0bb5d5ff1fb0ae97c20eee4c1 100644
--- a/sdk/lib/_internal/pub/test/snapshot/upgrades_snapshot_test.dart
+++ b/sdk/lib/_internal/pub/test/snapshot/upgrades_snapshot_test.dart
@@ -13,11 +13,13 @@ import '../test_pub.dart';
 main() {
   initConfig();
   integration("upgrades a snapshot when its package is upgraded", () {
-    servePackages([packageMap("foo", "1.2.3")], contents: [
-      d.dir("bin", [
-        d.file("hello.dart", "void main() => print('hello!');")
-      ])
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "1.2.3", contents: [
+        d.dir("bin", [
+          d.file("hello.dart", "void main() => print('hello!');")
+        ])
+      ]);
+    });
 
     d.appDir({"foo": "any"}).create();
 
@@ -27,11 +29,13 @@ main() {
       d.matcherFile('hello.dart.snapshot', contains('hello!'))
     ]).validate();
 
-    servePackages([packageMap("foo", "1.2.4")], contents: [
-      d.dir("bin", [
-        d.file("hello.dart", "void main() => print('hello 2!');")
-      ])
-    ]);
+    servePackages((builder) {
+      builder.serve("foo", "1.2.4", contents: [
+        d.dir("bin", [
+          d.file("hello.dart", "void main() => print('hello 2!');")
+        ])
+      ]);
+    });
 
     pubUpgrade(output: contains("Precompiled foo:hello."));