Only run [declareOutputs] once for each asset/transformer pair.

BUG=16442
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//227123007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@34892 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/barback/dart2js_transformer.dart

@@ -88,8 +88,10 @@ class Dart2JSTransformer extends Transformer implements LazyTransformer {
   Future declareOutputs(DeclaringTransform transform) {
     var primaryId = transform.primaryId;
     transform.declareOutput(primaryId.addExtension(".js"));
-    transform.declareOutput(primaryId.addExtension(".js.map"));
     transform.declareOutput(primaryId.addExtension(".precompiled.js"));
+    if (generateSourceMaps) {
+      transform.declareOutput(primaryId.addExtension(".js.map"));
+    }
     return new Future.value();
   }