Handle FutureOr type when inferring async closures.

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
index cd03355b581a5329284f969fda68387fd027291e..d6258de42710abf3ac131dd6e07f1cf4fd44cb6e 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
@@ -321,8 +321,13 @@ abstract class TypeInferrerImpl extends TypeInferrer {
   }
 
   DartType wrapFutureType(DartType type) {
+    var typeWithoutFutureOr = type;
+    if (type is InterfaceType &&
+        identical(type.classNode, coreTypes.futureOrClass)) {
+      typeWithoutFutureOr = type.typeArguments[0];
+    }
     return new InterfaceType(coreTypes.futureClass,
-        <DartType>[typeSchemaEnvironment.flattenFutures(type)]);
+        <DartType>[typeSchemaEnvironment.flattenFutures(typeWithoutFutureOr)]);
   }
 
   DartType wrapType(DartType type, Class class_) {