Fix typo in TypeInformationSystem.joinTypeMasks

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 4ff72abcc81347b0ada2d56bd23603fe6c047361..68cf701ff1467f387c9d2e877a8f1272abe7026a 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -553,7 +553,7 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
     }
 
     TypeMask newType = null;
-    for (TypeMask mask in masks) {
+    for (TypeMask mask in list) {
       newType = newType == null ? mask : newType.union(mask, classWorld);
       // Likewise - stop early if we already reach dynamic.
       if (newType.containsAll(classWorld)) return dynamicType;