Fix inference of the static type of an async function literal.

pkg/analyzer/lib/src/generated/static_type_analyzer.dart

Index: pkg/analyzer/lib/src/generated/static_type_analyzer.dart
diff --git a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
index 7b461df669cdc6f24e8fba46f306926a26b760f5..da3e45755b7b44f7c762e493c50aaff99be784b4 100644
--- a/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
+++ b/pkg/analyzer/lib/src/generated/static_type_analyzer.dart
@@ -1433,7 +1433,8 @@ class StaticTypeAnalyzer extends SimpleAstVisitor<Object> {
       type = _dynamicType;
     }
     if (body.isAsynchronous) {
-      return _typeProvider.futureType.substitute4(<DartType>[type]);
+      return _typeProvider.futureType.substitute4(
+          <DartType>[flattenFutures(_typeProvider, type)]);
     } else {
       return type;
     }