Reorder Future.then checks in StrongTypeSystemImpl.

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 f34e7e67fda6049ce5bf5af7d85e1c24fce63d83..7e4103d9e10266574e2830246831c51f9d151ecc 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -73,7 +73,6 @@ class StrongTypeSystemImpl extends TypeSystem {
   }
 
   /// Computes the greatest lower bound of [type1] and [type2].
-  @override
   DartType getGreatestLowerBound(
       TypeProvider provider, DartType type1, DartType type2) {
     // The greatest lower bound relation is reflexive.
@@ -208,16 +207,16 @@ class StrongTypeSystemImpl extends TypeSystem {
     // 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 (argumentTypes.isNotEmpty && argumentTypes[0] is FunctionType) {
+      Element element = fnType?.element;
+      bool isFutureThen = element is MethodElement &&
+          element.name == 'then' &&
+          element.enclosingElement.type.isDartAsyncFuture;
+      if (isFutureThen) {
+        // Ignore return context. We'll let the onValue function's return type
+        // drive inference.
+        returnContextType = null;
+      }
     }
 
     if (returnContextType != null) {