Stop emitting dart2js's .precompiled.js file.

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 6134e433784372acde83d22343690dc42bb23ac9..0200ac389806337f88e2bf1ff57a78e8ba18321e 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/dart2js_transformer.dart
@@ -89,7 +89,6 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
   void declareOutputs(DeclaringTransform transform) {
     var primaryId = transform.primaryId;
     transform.declareOutput(primaryId.addExtension(".js"));
-    transform.declareOutput(primaryId.addExtension(".precompiled.js"));
     if (_generateSourceMaps) {
       transform.declareOutput(primaryId.addExtension(".js.map"));
     }
@@ -284,6 +283,10 @@ class _BarbackCompilerProvider implements dart.CompilerProvider {
       return new NullSink<String>();
     }
 
+    // TODO(nweiz): remove this special case when dart2js stops generating these
+    // files.
+    if (extension.endsWith(".precompiled.js")) return new NullSink<String>();
+
     var primaryId = _transform.primaryInput.id;
 
     // Dart2js uses an empty string for the name of the entrypoint library.