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

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

+import 'dart:convert';
+import 'package:scheduled_test/scheduled_test.dart';
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+main() {
+  initConfig();
+  integration(
+      "compiles dart.js and interop.js next to entrypoints when "
+          "dartjs is explicitly configured",
+      () {
+    currentSchedule.timeout *= 3;
+    serve([d.dir('api', [d.dir('packages', [d.file('browser', JSON.encode({
+            'versions': [packageVersionApiMap(packageMap('browser', '1.0.0'))]
+          })),
+              d.dir(
+                  'browser',
+                  [
+                      d.dir(
+                          'versions',
+                          [
+                              d.file(
+                                  '1.0.0',
+                                  JSON.encode(
+                                      packageVersionApiMap(packageMap('browser', '1.0.0'), full: true)))])])])]),
+              d.dir(
+                  'packages',
+                  [
+                      d.dir(
+                          'browser',
+                          [
+                              d.dir(
+                                  'versions',
+                                  [
+                                      d.tar(
+                                          '1.0.0.tar.gz',
+                                          [
+                                              d.file('pubspec.yaml', yaml(packageMap("browser", "1.0.0"))),
+                                              d.dir(
+                                                  'lib',
+                                                  [
+                                                      d.file('dart.js', 'contents of dart.js'),
+                                                      d.file('interop.js', 'contents of interop.js')])])])])])]);
+    d.dir(appPath, [d.pubspec({
+        "name": "myapp",
+        "dependencies": {
+          "browser": "1.0.0"
+        },
+        "transformers": [{
+            "\$dart2js": {
+              "minify": true
+            }
+          }]
+      }),
+          d.dir(
+              'web',
+              [d.file('file.dart', 'void main() => print("hello");')])]).create();
+    pubGet();
+    schedulePub(
+        args: ["build"],
+        output: new RegExp(r'Built 4 files to "build".'),
+        exitCode: 0);
+    d.dir(
+        appPath,
+        [
+            d.dir(
+                'build',
+                [
+                    d.dir(
+                        'web',
+                        [
+                            d.matcherFile('file.dart.js', isMinifiedDart2JSOutput),
+                            d.matcherFile('file.dart.precompiled.js', isNot(isEmpty)),
+                            d.dir(
+                                'packages',
+                                [
+                                    d.dir(
+                                        'browser',
+                                        [
+                                            d.file('dart.js', 'contents of dart.js'),
+                                            d.file('interop.js', 'contents of interop.js')])])])])]).validate();
+  });
+}