Handle Creates and Returns annotations in KernelBehaviorBuilder

pkg/compiler/lib/src/kernel/world_builder.dart

Index: pkg/compiler/lib/src/kernel/world_builder.dart
diff --git a/pkg/compiler/lib/src/kernel/world_builder.dart b/pkg/compiler/lib/src/kernel/world_builder.dart
index 7f1f2ab749a0cbd2f6e60651ba394cb750fe0130..75c78c4598c0449e58974a8baf9e65906bcf1550 100644
--- a/pkg/compiler/lib/src/kernel/world_builder.dart
+++ b/pkg/compiler/lib/src/kernel/world_builder.dart
@@ -107,7 +107,10 @@ class KernelWorldBuilder extends KernelElementAdapterMixin {
   ClassEntity lookupClass(KLibrary library, String name) {
     KLibraryEnv libraryEnv = _libraryEnvs[library.libraryIndex];
     KClassEnv classEnv = libraryEnv.lookupClass(name);
-    return _getClass(classEnv.cls, classEnv);
+    if (classEnv != null) {
+      return _getClass(classEnv.cls, classEnv);
+    }
+    return null;
   }
 
   MemberEntity lookupClassMember(KClass cls, String name,