Remove code that requires whole-program compile

lib/src/transformer/asset_source.dart

-// Copyright (c) 2016, 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.
-
-import 'package:analyzer/src/generated/engine.dart';
-import 'package:analyzer/src/generated/source.dart' show Source, UriKind;
-import 'package:barback/barback.dart' show Asset, TimestampedData;
-import 'package:path/path.dart' as path;
-
-class AssetSource implements Source {
-  final Uri uri;
-  final Asset asset;
-  final String contentString;
-  AssetSource(this.uri, this.asset, this.contentString);
-
-  @override
-  toString() => 'AssetSource($uri, ${asset.id})';
-
-  @override
-  TimestampedData<String> get contents =>
-      new TimestampedData(modificationStamp, contentString);
-
-  @override
-  String get encoding => null;
-
-  @override
-  bool exists() => true;
-
-  @override
-  String get fullName => uri.toString();
-
-  @override
-  bool get isInSystemLibrary => uriKind == UriKind.DART_URI;
-
-  @override
-  int get modificationStamp => 0;
-
-  @override
-  Uri resolveRelativeUri(Uri relativeUri) {
-    var resolvedPath = path.join(path.dirname(uri.path), relativeUri.path);
-    return new Uri(scheme: uri.scheme, path: resolvedPath);
-  }
-
-  @override
-  String get shortName => uri.toString();
-
-  @override
-  Source get source => this;
-
-  @override
-  UriKind get uriKind {
-    switch (uri.scheme) {
-      case 'package':
-        return UriKind.PACKAGE_URI;
-
-      case 'dart':
-        return UriKind.DART_URI;
-
-      case 'file':
-        return UriKind.FILE_URI;
-
-      default:
-        throw new StateError(uri.toString());
-    }
-  }
-}