fix #25486, promote from dynamic in strong mode

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 911dfa9f81e072ddd26c99fbd157abb18979fa3a..5eb00ee385d3c67024d4c5f5bd9afdc82ecf54d9 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -29,6 +29,9 @@ class StrongTypeSystemImpl implements TypeSystem {
     return ft.parameters.any((p) => predicate(p.type));
   }
 
+  @override
+  bool canPromoteToType(DartType to, DartType from) => isSubtypeOf(to, from);
+
   /**
    * Given a type t, if t is an interface type with a call method
    * defined, return the function type for the call method, otherwise
@@ -563,6 +566,16 @@ class StrongTypeSystemImpl implements TypeSystem {
  */
 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.
+   *
+   * In strong mode, this is equivalent to [isSubtypeOf].
+   */
+  bool canPromoteToType(DartType to, DartType from);
+
+  /**
    * Compute the least upper bound of two types.
    */
   DartType getLeastUpperBound(
@@ -606,6 +619,14 @@ class TypeSystemImpl implements TypeSystem {
   TypeSystemImpl();
 
   @override
+  bool canPromoteToType(DartType to, DartType from) {
+    // Declared type should not be "dynamic".
+    // Promoted type should not be "dynamic".
+    // Promoted type should be more specific than declared.
+    return !from.isDynamic && !to.isDynamic && to.isMoreSpecificThan(from);
+  }
+
+  @override
   DartType getLeastUpperBound(
       TypeProvider typeProvider, DartType type1, DartType type2) {
     // The least upper bound relation is reflexive.