Provide URLs for assets from lib/ and asset/ in pub's websocket API.

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

Index: sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
index 77328fbdeac137b29e776f2a28a1622226d54e97..97d00cd6601ab1c04f15d11f6f76989de9e9df02 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -33,10 +33,10 @@ Future<Set> loadTransformers(BuildEnvironment environment,
     // TODO(nweiz): load from a "package:" URI when issue 12474 is fixed.
 
     var baseUrl = transformerServer.url;
-    var uri = '$baseUrl/packages/${id.package}/$path';
+    var uri = baseUrl.resolve('packages/${id.package}/$path');
     var code = 'import "$uri";\n' +
         readAsset(p.join("dart", "transformer_isolate.dart"))
-            .replaceAll('<<URL_BASE>>', baseUrl);
+            .replaceAll('<<URL_BASE>>', baseUrl.toString());
     log.fine("Loading transformers from $assetId");
 
     var port = new ReceivePort();
@@ -44,7 +44,7 @@ Future<Set> loadTransformers(BuildEnvironment environment,
         .then((_) => port.first)
         .then((sendPort) {
       return _call(sendPort, {
-        'library': uri,
+        'library': uri.toString(),
         'mode': environment.mode.name,
         // TODO(nweiz): support non-JSON-encodable configuration maps.
         'configuration': JSON.encode(id.configuration)