Always pass absolute paths to dart2js.

lib/src/barback/dart2js_transformer.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.dart2js_transformer;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:analyzer/analyzer.dart';
@@ skipping 182 matching lines @@
   }
 }
 
 /// Defines an interface for dart2js to communicate with barback and pub.
 ///
 /// Note that most of the implementation of diagnostic handling here was
 /// copied from [FormattingDiagnosticHandler] in dart2js. The primary
 /// 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;
   String _libraryRootPath;
 
   /// The map of previously loaded files.
   ///
   /// Used to show where an error occurred in a source file.
   final _sourceFiles = new Map<String, SourceFile>();
 
@@ skipping 196 matching lines @@
   }
 }
 
 /// An [EventSink] that discards all data. Provided to dart2js when we don't
 /// want an actual output.
 class NullSink<T> implements EventSink<T> {
   void add(T event) {}
   void addError(errorEvent, [StackTrace stackTrace]) {}
   void close() {}
 }