Support loading transformer plugins from pub.

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

Index: sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..66fda923d47c2257153737685d5a33dc2b3f346e
--- /dev/null
+++ b/sdk/lib/_internal/pub/lib/src/barback/rewrite_import_transformer.dart
@@ -0,0 +1,45 @@
+// 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.rewrite_import_transformer;
+
+import 'package:barback/barback.dart';
+import 'package:analyzer_experimental/analyzer.dart';
+
+/// A transformer used internally to rewrite "package:" imports so they point to
+/// the barback server rather than to pub's package root.
+class RewriteImportTransformer extends Transformer {
+  String get allowedExtensions => '.dart';
+
+  Future apply(Transform transform) {
+    return transform.primaryInput.readAsString().then((contents) {
+      var collector = new _DirectiveCollector();
+      parseCompilationUnit(contents, name: transform.primaryInput.id.toString())
+          .accept(collector);
+
+      var buffer = new StringBuffer();
+      var index = 0;
+      for (var directive in collector.directives) {
+        var uri = Uri.parse(directive.uri.stringValue);
+        if (uri.scheme != 'package') continue;
+
+        buffer
+          ..write(contents.substring(index, directive.uri.literal.offset))
+          ..write('"/packages/${uri.path}"');
+        index = directive.uri.literal.end;
+      }
+      buffer.write(contents.substring(index, contents.length));
+
+      transform.addOutput(
+          new Asset.fromString(transform.primaryInput.id, buffer.toString()));
+    });
+  }
+}
+
+/// A simple visitor that collects import and export nodes.
+class _DirectiveCollector extends GeneralizingASTVisitor {
+  final directives = <UriBasedDirective>[];
+
+  visitUriBasedDirective(UriBasedDirective node) => directives.add(node);
+}