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

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.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 file.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 444 matching lines @@
       // `async`, `async*`, or `sync*`.
       if (isGenerator) {
         if (isAsync) {
           inferredReturnType = inferrer.wrapType(
               inferredReturnType, inferrer.coreTypes.streamClass);
         } else {
           inferredReturnType = inferrer.wrapType(
               inferredReturnType, inferrer.coreTypes.iterableClass);
         }
       } else if (isAsync) {
-        inferredReturnType = inferrer.wrapType(
-            inferredReturnType, inferrer.coreTypes.futureClass);
+        inferredReturnType = inferrer.wrapFutureType(inferredReturnType);
       }
     }
 
     // Then the result of inference is `<T0, ..., Tn>(R0 x0, ..., Rn xn) B` with
     // type `<T0, ..., Tn>(R0, ..., Rn) -> M’` (with some of the `Ri` and `xi`
     // denoted as optional or named parameters, if appropriate).
     if (needToSetReturnType) {
       inferrer.instrumentation?.record(Uri.parse(inferrer.uri), fileOffset,
           'returnType', new InstrumentationValueForType(inferredReturnType));
       function.returnType = inferredReturnType;
@@ skipping 813 matching lines @@
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     var variable = this.variable as KernelVariableDeclaration;
     typeNeeded =
         inferrer.listener.variableSetEnter(this, typeContext) || typeNeeded;
     var inferredType =
         inferrer.inferExpression(value, variable._declaredType, typeNeeded);
     inferrer.listener.variableSetExit(this, inferredType);
     return inferredType;
   }
 }