fix warning in polymer script_compactor

pkg/polymer/lib/src/build/script_compactor.dart

Index: pkg/polymer/lib/src/build/script_compactor.dart
diff --git a/pkg/polymer/lib/src/build/script_compactor.dart b/pkg/polymer/lib/src/build/script_compactor.dart
index 7606730b39f5a0c8e0d5b8ee2fc8c19c1b35ea0d..bfb2c5623976f583bcab147b7d23e1065a5e04f8 100644
--- a/pkg/polymer/lib/src/build/script_compactor.dart
+++ b/pkg/polymer/lib/src/build/script_compactor.dart
@@ -42,7 +42,7 @@ class ScriptCompactor extends Transformer with PolymerTransformer {
     var logger = transform.logger;
     return readPrimaryAsHtml(transform).then((document) {
       return transform.readInputAsString(secondaryId).then((libraryIds) {
-        var libraries = JSON.decode(libraryIds).map(
+        var libraries = (JSON.decode(libraryIds) as Iterable).map(
           (data) => new AssetId.deserialize(data)).toList();
         var mainLibraryId;
         var mainScriptTag;