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

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

+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+import 'utils.dart';
+main() {
+  initConfig();
+  integration(
+      "compiles dart.js and interop.js next to entrypoints when "
+          "browser is a dev dependency",
+      () {
+    currentSchedule.timeout *= 3;
+    serveBrowserPackage();
+    d.dir(appPath, [d.pubspec({
+        "name": "myapp",
+        "dev_dependencies": {
+          "browser": "any"
+        }
+      }),
+          d.dir(
+              'web',
+              [d.file('file.dart', 'void main() => print("hello");')])]).create();
+    pubGet();
+    schedulePub(
+        args: ["build", "--all"],
+        output: new RegExp(r'Built 4 files to "build".'));
+    d.dir(
+        appPath,
+        [
+            d.dir(
+                'build',
+                [
+                    d.dir(
+                        'web',
+                        [
+                            d.dir(
+                                'packages',
+                                [
+                                    d.dir(
+                                        'browser',
+                                        [
+                                            d.file('dart.js', 'contents of dart.js'),
+                                            d.file('interop.js', 'contents of interop.js')])])])])]).validate();
+  });
+}