Improve barback/pub logging.

sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart

Index: sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
index 9405f43d4c3d56f9e8a574deba665ca0158bdb7a..ef5185ed6e78f93998f4d81a0ffdd3277723ff28 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -41,7 +41,14 @@ class Dart2JSTransformer extends Transformer {
 
     return transform.primaryInput.readAsString().then((code) {
       try {
-        if (!dart.isEntrypoint(parseCompilationUnit(code))) return;
+        var id = transform.primaryInput.id;
+        var name = id.path;
+        if (id.package != _graph.entrypoint.root.name) {
+          name += " in ${id.package}";
+        }
+
+        var parsed = parseCompilationUnit(code, name: name);
+        if (!dart.isEntrypoint(parsed)) return;
       } on AnalyzerErrorGroup catch (e) {
         transform.logger.error(e.message);
         return;
@@ -64,6 +71,12 @@ class Dart2JSTransformer extends Transformer {
           packageRoot: packageRoot,
           inputProvider: provider.readStringFromUri,
           diagnosticHandler: provider.handleDiagnostic).then((js) {
+        if (js == null) {
+          // The compile failed and errors should have already been reported
+          // through the diagnostic handler, so just do nothing here.
+          return;
+        }
+
         var id = transform.primaryInput.id.changeExtension(".dart.js");
         transform.addOutput(new Asset.fromString(id, js));