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 71996f775ba848c391370757a8298b48ba0128ed..cd03355b581a5329284f969fda68387fd027291e 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
@@ -320,6 +320,11 @@ 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]);
   }