Fix CPA inference.

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

Index: pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
index 8a7e2b2d72c8c374784e489615e3d8b697db5c32..4f952146517052a087a3c66a7363b3057c792484 100644
--- a/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/concrete_types_inferrer.dart
@@ -1201,7 +1201,10 @@ class ConcreteTypesInferrer
         jsArrayClass.lookupMember('removeLast');
     List<String> typePreservingOps = const ['+', '-', '*'];
     listConstructor =
-        compiler.listClass.lookupDefaultConstructor().implementation;
+        compiler.listClass.lookupDefaultConstructor();
+    if (listConstructor != null) {
+      listConstructor = listConstructor.implementation;
+    }
     emptyConcreteType = new ConcreteType.empty(compiler.maxConcreteTypeSize,
                                                baseTypes);
     nullConcreteType = singletonConcreteType(const NullBaseType());