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

sdk/lib/_internal/pub_generated/test/build/copies_non_dart_files_to_build_test.dart

+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration("copies non-Dart files to build/", () {
+    servePackages((builder) => builder.serve("browser", "1.0.0"));
+    d.dir(appPath, [d.appPubspec({
+        "browser": "1.0.0"
+      }),
+          d.dir(
+              'web',
+              [
+                  d.file('file.txt', 'contents'),
+                  d.dir('subdir', [d.file('subfile.txt', 'subcontents')])])]).create();
+    schedulePub(
+        args: ["build"],
+        output: new RegExp(r'Built 2 files to "build".'));
+    d.dir(
+        appPath,
+        [
+            d.dir(
+                'build',
+                [
+                    d.dir(
+                        'web',
+                        [
+                            d.nothing('packages'),
+                            d.file('file.txt', 'contents'),
+                            d.dir('subdir', [d.file('subfile.txt', 'subcontents')])])])]).validate();
+  });
+}