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

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

Index: pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
index 202293e7b048997012a4c603921dacce00e2008d..e0d802d84a923d4f82a165e824e3c07bcda985d1 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_schema_environment.dart
@@ -49,6 +49,28 @@ class TypeSchemaEnvironment extends TypeEnvironment {
     constraint.upper = getGreatestLowerBound(constraint.upper, upper);
   }
 
+  /// Implements the function "flatten" defined in the spec, where T is [type]:
+  ///
+  ///     If T = Future<S> then flatten(T) = flatten(S).
+  ///
+  ///     Otherwise if T <: Future then let S be a type such that T << Future<S>
+  ///     and for all R, if T << Future<R> then S << R.  Then flatten(T) = S.
+  ///
+  ///     In any other circumstance, flatten(T) = T.
+  DartType flattenFutures(DartType type) {
+    if (type is InterfaceType) {
+      if (identical(type.classNode, coreTypes.futureClass)) {
+        return flattenFutures(type.typeArguments[0]);
+      }
+      InterfaceType futureBase =
+          hierarchy.getTypeAsInstanceOf(type, coreTypes.futureClass);
+      if (futureBase != null) {
+        return futureBase.typeArguments[0];
+      }
+    }
+    return type;
+  }
+
   /// Computes the greatest lower bound of [type1] and [type2].
   DartType getGreatestLowerBound(DartType type1, DartType type2) {
     // The greatest lower bound relation is reflexive.  Note that we don't test