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

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

 // Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 import 'package:front_end/src/base/instrumentation.dart';
 import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart';
 import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart';
 import 'package:front_end/src/fasta/type_inference/type_inference_listener.dart';
 import 'package:front_end/src/fasta/type_inference/type_promotion.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
@@ skipping 302 matching lines @@
     if (isExpressionFunction &&
         returnType is InterfaceType &&
         identical(returnType.classNode, coreTypes.nullClass)) {
       // Analyzer coerces `Null` to `dynamic` in expression functions; the spec
       // doesn't say to do this.  TODO(paulberry): resolve this difference.
       return const DynamicType();
     }
     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]);
   }
 
   void _forEachArgument(
       Arguments arguments, void callback(String name, Expression expression)) {
     for (var expression in arguments.positional) {
       callback(null, expression);
     }
     for (var namedExpression in arguments.named) {
       callback(namedExpression.name, namedExpression.value);
     }
   }
 }