fix #25944, improve Future.then inference

pkg/analyzer/lib/dart/element/type.dart

Index: pkg/analyzer/lib/dart/element/type.dart
diff --git a/pkg/analyzer/lib/dart/element/type.dart b/pkg/analyzer/lib/dart/element/type.dart
index 11c97e8d03b784a0c03045c0ff679983a79eb839..29524d701dbf24f8ef4a3e7fcc49d03d17be187e 100644
--- a/pkg/analyzer/lib/dart/element/type.dart
+++ b/pkg/analyzer/lib/dart/element/type.dart
@@ -24,14 +24,15 @@ library analyzer.dart.element.type;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/dart/element/type.dart' show InterfaceTypeImpl;
-import 'package:analyzer/src/generated/type_system.dart' show TypeSystem;
+import 'package:analyzer/src/generated/type_system.dart'
+    show TypeContext, TypeSystem;
 
 /**
  * The type associated with elements in the element model.
  *
  * Clients may not extend, implement or mix-in this class.
  */
-abstract class DartType {
+abstract class DartType implements TypeContext {

[Leaf, 2016/08/05 22:32:35]

It bothers me a bit that this leaks into the public API, even though it's just a
marker.  It seems like it would work to make FutureUnion type a subclass of
TypeImpl instead.  That's probably a little annoying since there's a bunch of
API surface from DartType that doesn't really apply to TypeContext.  But at
least it keeps it behind the API boundary.

Maybe an alternative would be to provide for this in a more abstract way by
replacing TypeContext here with something more abstract like "PreType" or
"TypeSchema" or "Classifier" or "TypeData" or something.  Then TypeContext could
be a private implementation of this, and if we want to use this trick again we
don't have to change this again?

WDYT?

[Jennifer Messerly, 2016/08/08 21:59:26]

Yeah, making it a subtype of TypeImpl works for me. Let me give that a try.

   /**
    * An empty list of types.
    */
@@ -290,7 +291,7 @@ abstract class FunctionType implements ParameterizedType {
    *
    * In addition, the following subtype rules apply:
    *
-   * <i>(T<sub>1</sub>, &hellip;, T<sub>n</sub>, []) &rarr; T <: (T<sub>1</sub>,
+   * <i>(T<sub>1</sub>, &hellip;, T<sub>n</sub>, `[]`) &rarr; T <: (T<sub>1</sub>,

[Leaf, 2016/08/05 22:32:35]

line length.

[Jennifer Messerly, 2016/08/08 21:59:26]

Done.

    * &hellip;, T<sub>n</sub>) &rarr; T.</i><br>
    * <i>(T<sub>1</sub>, &hellip;, T<sub>n</sub>) &rarr; T <: (T<sub>1</sub>,
    * &hellip;, T<sub>n</sub>, {}) &rarr; T.</i><br>