Store the async-await compiled pub code directly in the repo.

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

+library pub_tests;
+import 'package:path/path.dart' as p;
+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration("upgrades a snapshot when its package is upgraded", () {
+    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();
+    pubGet(output: contains("Precompiled foo:hello."));
+    d.dir(
+        p.join(appPath, '.pub', 'bin', 'foo'),
+        [d.matcherFile('hello.dart.snapshot', contains('hello!'))]).validate();
+    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."));
+    d.dir(
+        p.join(appPath, '.pub', 'bin', 'foo'),
+        [d.matcherFile('hello.dart.snapshot', contains('hello 2!'))]).validate();
+    var process = pubRun(args: ['foo:hello']);
+    process.stdout.expect("hello 2!");
+    process.shouldExit();
+  });
+}