Add a test for pub's async_await compiler.

sdk/lib/_internal/pub/lib/src/io.dart

Index: sdk/lib/_internal/pub/lib/src/io.dart
diff --git a/sdk/lib/_internal/pub/lib/src/io.dart b/sdk/lib/_internal/pub/lib/src/io.dart
index 58a52ef00eaf0e412fb618ec23763f836da0273f..abf9531dc9305485c3360ea80caececb01881cea 100644
--- a/sdk/lib/_internal/pub/lib/src/io.dart
+++ b/sdk/lib/_internal/pub/lib/src/io.dart
@@ -485,11 +485,13 @@ String assetPath(String target) {
     return path.join(
         sdk.rootDirectory, 'lib', '_internal', 'pub', 'asset', target);
   } else {
-    return path.join(
-        path.dirname(libraryPath('pub.io')), '..', '..', 'asset', target);
+    return path.join(pubRoot, 'asset', target);
   }
 }
 
+/// Returns the path to the root of pub's sources in the Dart repo.
+String get pubRoot => path.join(repoRoot, 'sdk', 'lib', '_internal', 'pub');
+
 /// Returns the path to the root of the Dart repository.
 ///
 /// This throws a [StateError] if it's called when running pub from the SDK.