fix #25486, promote from dynamic in strong mode

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 /// General type checking tests
 library analyzer.test.src.task.strong.checker_test;
 
 import 'package:unittest/unittest.dart';
@@ skipping 1414 matching lines @@
             bar(y: 1, x: 2, z: 3);
             String x = bar(z: '1', x: '2', y: '3');
             bar(y: 1);
             x = bar(x: '1', z: 42);
             x = /*severe:STATIC_TYPE_ERROR*/bar(y: 1, x: 2, z: 3);
             x = /*severe:STATIC_TYPE_ERROR*/bar(x: 1);
           }
       '''
   });
 
+  testChecker('type promotion from dynamic', {
+    '/main.dart': r'''
+          f() {
+            dynamic x;
+            if (x is int) {
+              int y = x;
+              String z = /*severe:STATIC_TYPE_ERROR*/x;
+            }
+          }
+          g() {
+            Object x;
+            if (x is int) {
+              int y = x;
+              String z = /*severe:STATIC_TYPE_ERROR*/x;
+            }
+          }
+    '''
+  });
+
   testChecker('unary operators', {
     '/main.dart': '''
       class A {
         A operator ~() {}
         A operator +(int x) {}
         A operator -(int x) {}
         A operator -() {}
       }
 
       foo() => new A();
@@ skipping 1088 matching lines @@
 
         baz1() sync* { yield* (/*info:DYNAMIC_CAST*/x); }
         Iterable baz2() sync* { yield* (/*info:DYNAMIC_CAST*/x); }
         Iterable<int> baz3() sync* { yield* (/*warning:DOWN_CAST_COMPOSITE*/x); }
         Iterable<int> baz4() sync* { yield* new Iterable<int>(); }
         Iterable<int> baz5() sync* { yield* (/*info:INFERRED_TYPE_ALLOCATION*/new Iterable()); }
     '''
     });
   });
 }