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

sdk/lib/_internal/pub_generated/test/dart2js/source_maps_include_core_libs_in_subdirectory_test.dart

+import 'package:path/path.dart' as path;
+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../serve/utils.dart';
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration(
+      "Dart core libraries are available to source maps when the "
+          "build directory is a subdirectory",
+      () {
+    d.dir(
+        appPath,
+        [
+            d.appPubspec(),
+            d.dir(
+                "web",
+                [
+                    d.dir(
+                        "sub",
+                        [
+                            d.file(
+                                "main.dart",
+                                "main() => new StringBuffer().writeAll(['s']);")])])]).create();
+    var webSub = path.join("web", "sub");
+    pubServe(args: [webSub]);
+    requestShouldSucceed(
+        "main.dart.js.map",
+        contains(r"packages/$sdk/lib/core/string_buffer.dart"),
+        root: webSub);
+    requestShouldSucceed(
+        r"packages/$sdk/lib/core/string_buffer.dart",
+        contains("class StringBuffer"),
+        root: webSub);
+    endPubServe();
+  });
+}