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

sdk/lib/_internal/pub_generated/test/serve/serve_from_dependency_lib_test.dart

+library pub_tests;
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+main() {
+  initConfig();
+  integration("'packages' URLs look in the dependency's lib directory", () {
+    d.dir(
+        "foo",
+        [
+            d.libPubspec("foo", "0.0.1"),
+            d.dir(
+                "lib",
+                [
+                    d.file("lib.dart", "foo() => 'foo';"),
+                    d.dir("sub", [d.file("lib.dart", "bar() => 'bar';")])])]).create();
+    d.dir(appPath, [d.appPubspec({
+        "foo": {
+          "path": "../foo"
+        }
+      })]).create();
+    pubServe(shouldGetFirst: true);
+    requestShouldSucceed("packages/foo/lib.dart", "foo() => 'foo';");
+    requestShouldSucceed("packages/foo/sub/lib.dart", "bar() => 'bar';");
+    requestShouldSucceed("foo/packages/foo/lib.dart", "foo() => 'foo';");
+    requestShouldSucceed("a/b/packages/foo/sub/lib.dart", "bar() => 'bar';");
+    endPubServe();
+  });
+}