Make pub strong-mode clean.

lib/src/barback.dart

Index: lib/src/barback.dart
diff --git a/lib/src/barback.dart b/lib/src/barback.dart
index 6fd624588c94f045be4b94cfc6b67e9af1d475fd..a13ec89b3aa6b26931c0bcd579f250a9eb64585a 100644
--- a/lib/src/barback.dart
+++ b/lib/src/barback.dart
@@ -37,7 +37,8 @@ import 'package:pub_semver/pub_semver.dart';
 final pubConstraints = {
   "barback": new VersionConstraint.parse(">=0.15.0 <0.15.3"),
   "source_span": new VersionConstraint.parse(">=1.0.0 <2.0.0"),
-  "stack_trace": new VersionConstraint.parse(">=0.9.1 <2.0.0")
+  "stack_trace": new VersionConstraint.parse(">=0.9.1 <2.0.0"),
+  "async": new VersionConstraint.parse(">=1.8.0 <2.0.0")
 };
 
 /// Converts [id] to a "package:" URI.