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

sdk/lib/_internal/pub_generated/test/upgrade/git/upgrade_to_nonexistent_pubspec_test.dart

+library pub_tests;
+import '../../descriptor.dart' as d;
+import '../../test_pub.dart';
+main() {
+  initConfig();
+  integration("upgrades Git packages to a nonexistent pubspec", () {
+    ensureGit();
+    var repo =
+        d.git('foo.git', [d.libDir('foo'), d.libPubspec('foo', '1.0.0')]);
+    repo.create();
+    d.appDir({
+      "foo": {
+        "git": "../foo.git"
+      }
+    }).create();
+    pubGet();
+    d.dir(
+        packagesPath,
+        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
+    repo.runGit(['rm', 'pubspec.yaml']);
+    repo.runGit(['commit', '-m', 'delete']);
+    pubUpgrade(
+        error: new RegExp(
+            r'Could not find a file named "pubspec.yaml" ' r'in "[^\n]*"\.'));
+    d.dir(
+        packagesPath,
+        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
+  });
+}