fixes #25482, flatten Futures in strong mode so Future.then works

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 5eb00ee385d3c67024d4c5f5bd9afdc82ecf54d9..b85205c14911c99f52cac721d098a731e4500744 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -32,6 +32,9 @@ class StrongTypeSystemImpl implements TypeSystem {
   @override
   bool canPromoteToType(DartType to, DartType from) => isSubtypeOf(to, from);
 
+  @override
+  bool isMoreSpecificThan(DartType t1, DartType t2) => isSubtypeOf(t1, t2);
+
   /**
    * Given a type t, if t is an interface type with a call method
    * defined, return the function type for the call method, otherwise
@@ -569,13 +572,22 @@ abstract class TypeSystem {
    * Returns `true` if we can promote to the first type from the second type.
    *
    * In the standard Dart type system, it is not possible to promote from or to
-   * `dynamic`, and we must be promoting to a more specific type.
+   * `dynamic`, and we must be promoting to a more specific type, see
+   * [isMoreSpecificThan].
    *
    * In strong mode, this is equivalent to [isSubtypeOf].
    */
   bool canPromoteToType(DartType to, DartType from);
 
   /**
+   * Return `true` if the [leftType] is more specific than the [rightType]
+   * (that is, if leftType << rightType), as defined in the Dart language spec.
+   *
+   * In strong mode, this is equivalent to [isSubtypeOf].
+   */
+  bool isMoreSpecificThan(DartType leftType, DartType rightType);
+
+  /**
    * Compute the least upper bound of two types.
    */
   DartType getLeastUpperBound(
@@ -619,6 +631,10 @@ class TypeSystemImpl implements TypeSystem {
   TypeSystemImpl();
 
   @override
+  bool isMoreSpecificThan(DartType t1, DartType t2) =>
+      t1.isMoreSpecificThan(t2);
+
+  @override
   bool canPromoteToType(DartType to, DartType from) {
     // Declared type should not be "dynamic".
     // Promoted type should not be "dynamic".