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

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index d959c7e4ed10ba9370ee392a740c7d0b73ded450..2003ab312c6ca0c61b17fd7abe3d2a18592b2089 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -462,8 +462,7 @@ class KernelFunctionExpression extends FunctionExpression
               inferredReturnType, inferrer.coreTypes.iterableClass);
         }
       } else if (isAsync) {
-        inferredReturnType = inferrer.wrapType(
-            inferredReturnType, inferrer.coreTypes.futureClass);
+        inferredReturnType = inferrer.wrapFutureType(inferredReturnType);
       }
     }