Handle null in augmentField

sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart b/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
index 1d8731d2a19aaa59156709536720465c44e3462f..9e30e5877a98f21d9ca8592818a8c9d15b610071 100644
--- a/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
@@ -1037,10 +1037,12 @@ class TypeInferrerVisitor extends ResolvedVisitor<ConcreteType> {
         augmentField(baseReceiverType, member);
       }
     } else {
-      for (ClassBaseType baseReceiverType in receiverType.baseTypes) {
-        Element member = baseReceiverType.element.lookupMember(source);
+      for (BaseType baseReceiverType in receiverType.baseTypes) {
+        if (!baseReceiverType.isClass()) continue;
+        ClassBaseType baseReceiverClassType = baseReceiverType;
+        Element member = baseReceiverClassType.element.lookupMember(source);
         if (member != null) {
-          augmentField(baseReceiverType, member);
+          augmentField(baseReceiverClassType, member);
         }
       }
     }