Revert "Flatten futures when inferring the type of an async function expression."

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..71996f775ba848c391370757a8298b48ba0128ed 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
@@ -320,11 +320,6 @@ abstract class TypeInferrerImpl extends TypeInferrer {
     return returnType;
   }
 
-  DartType wrapFutureType(DartType type) {
-    return new InterfaceType(coreTypes.futureClass,
-        <DartType>[typeSchemaEnvironment.flattenFutures(type)]);
-  }
-
   DartType wrapType(DartType type, Class class_) {
     return new InterfaceType(class_, <DartType>[type]);
   }