Use real "package:" imports in pub's plugin-loading code.

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 7f81922b5f78020802bc81e1730fcecbe4be3bda..70b6151f0bc974da8b6e2d15327e03617d77969b 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/load_transformers.dart
@@ -34,9 +34,15 @@ Future<Set> loadTransformers(BuildEnvironment environment,
 
     var baseUrl = transformerServer.url;
     var uri = '$baseUrl/packages/${id.package}/$path';
-    var code = 'import "$uri";\n' +
-        readAsset(p.join("dart", "transformer_isolate.dart"))
-            .replaceAll('<<URL_BASE>>', baseUrl);
+    var code = """
+        import 'dart:isolate';
+
+        import '$uri';
+
+        import r'$baseUrl/packages/\$pub/transformer_isolate.dart';
+
+        void main(_, SendPort replyTo) => loadTransformers(replyTo);
+        """;
     log.fine("Loading transformers from $assetId");
 
     var port = new ReceivePort();
@@ -60,7 +66,7 @@ Future<Set> loadTransformers(BuildEnvironment environment,
       if (error.type != 'IsolateSpawnException') throw error;
       // TODO(nweiz): don't parse this as a string once issues 12617 and 12689
       // are fixed.
-      if (!error.message.split('\n')[1].startsWith('import "$uri";')) {
+      if (!error.message.split('\n')[1].endsWith("import '$uri';")) {
         throw error;
       }
 
@@ -148,6 +154,11 @@ Map _serializeTransform(Transform transform) {
         return null;
       }
 
+      if (message['type'] == 'consumePrimary') {
+        transform.consumePrimary();
+        return null;
+      }
+
       assert(message['type'] == 'log');
       var method;
       if (message['level'] == 'Info') {