Improve Future<T>.then<R> inference

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

Index: pkg/analyzer/lib/src/generated/type_system.dart
diff --git a/pkg/analyzer/lib/src/generated/type_system.dart b/pkg/analyzer/lib/src/generated/type_system.dart
index 19b5be5fdb2120fcfec3753681271abc4210e954..f34e7e67fda6049ce5bf5af7d85e1c24fce63d83 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -203,6 +203,23 @@ class StrongTypeSystemImpl extends TypeSystem {
     var inferringTypeSystem =
         new _StrongInferenceTypeSystem(typeProvider, fnType.typeFormals);
 
+    // Special case inference for Future.then.
+    //
+    // We don't have union types, so Future<T>.then<S> is typed to take a
+    // callback `T -> S`. However, the lambda might actually return a
+    // Future<S>. So we handle that special case here.
+    Element element = fnType?.element;
+    bool isFutureThen = element is MethodElement &&
+        element.name == 'then' &&
+        element.enclosingElement.type.isDartAsyncFuture;
+    if (isFutureThen &&
+        argumentTypes.isNotEmpty &&
+        argumentTypes[0] is FunctionType) {
+      // Ignore return context. We'll let the onValue function's return type
+      // drive inference.
+      returnContextType = null;
+    }
+
     if (returnContextType != null) {
       inferringTypeSystem.isSubtypeOf(fnType.returnType, returnContextType);
     }