Always pass absolute paths to dart2js.

lib/src/barback/dart2js_transformer.dart

Index: lib/src/barback/dart2js_transformer.dart
diff --git a/lib/src/barback/dart2js_transformer.dart b/lib/src/barback/dart2js_transformer.dart
index 32f3e4e6327724a1cfca8194d16227fb0246198d..abcc20cda98a559ed16aa8ef3b08d9f864ecad90 100644
--- a/lib/src/barback/dart2js_transformer.dart
+++ b/lib/src/barback/dart2js_transformer.dart
@@ -200,7 +200,8 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
 /// difference is that it uses barback's logging code and, more importantly, it
 /// handles missing source files more gracefully.
 class _BarbackCompilerProvider implements dart.CompilerProvider {
-  Uri get libraryRoot => Uri.parse("${path.toUri(_libraryRootPath)}/");
+  Uri get libraryRoot =>
+      Uri.parse("${path.toUri(path.absolute(_libraryRootPath))}/");
 
   final AssetEnvironment _environment;
   final Transform _transform;