Handle null in augmentField

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of types;
 
 class CancelTypeInferenceException {
   final Node node;
   final String reason;
 
@@ skipping 1019 matching lines @@
       }
     }
 
     if (receiverType.isUnkown()) {
       for (FunctionElement member in inferrer.getMembersByName(source)) {
         Element classElem = member.getEnclosingClass();
         BaseType baseReceiverType = new ClassBaseType(classElem);
         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);
         }
       }
     }
     return argumentType;
   }
 
   SourceString canonicalizeCompoundOperator(String op) {
     // TODO(ahe): This class should work on elements or selectors, not
     // names.  Otherwise, it is repeating work the resolver has
     // already done (or should have done).  In this case, the problem
@@ skipping 399 matching lines @@
   }
 
   void internalError(String reason, {Node node}) {
     inferrer.fail(node, reason);
   }
 
   ConcreteType visitTypeReferenceSend(Send) {
     return new ConcreteType.singleton(inferrer.baseTypes.typeBaseType);
   }
 }