Add the ability for a PackageProvider to declare static packages.

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

Index: sdk/lib/_internal/pub/lib/src/barback.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback.dart b/sdk/lib/_internal/pub/lib/src/barback.dart
index 6ea65f7798828e125ebc8009c07a1e017f58e37d..fb4ff51d27cf60d2c81a31c4e3098015830dcd2e 100644
--- a/sdk/lib/_internal/pub/lib/src/barback.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback.dart
@@ -38,7 +38,7 @@ import 'version.dart';
 ///
 /// [compat]: https://gist.github.com/nex3/10942218
 final pubConstraints = {
-  "barback": new VersionConstraint.parse(">=0.13.0 <0.15.1"),
+  "barback": new VersionConstraint.parse(">=0.13.0 <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")
 };