support `@virtual` fields, fix #27384

CHANGELOG.md

 ## 1.20.0
 
 ### Dart VM
 
 * We have improved the way that the VM locates the native code library for a
   native extension (e.g. `dart-ext:` import). We have updated this
   [article on native extensions](https://www.dartlang.org/articles/dart-vm/native-extensions)
   to reflect the VM's improved behavior.
 
 * Linux builds of the VM will now use the tcmalloc library for memory
@@ skipping 55 matching lines @@
       // this override is legal, it will check at runtime if we actually
       // got a MyView.
       addChild(@checked MyView v) {}
     }
     main() {
       dynamic mv = new MyView();
       mv.addChild(new View()); // runtime error
     }
     ```
 
+* New feature - use `@virtual` to allow field overrides in strong mode
+    (SDK issue [27384](https://github.com/dart-lang/sdk/issues/27384)).
+
+    ```dart
+    import 'package:meta/meta.dart' show virtual;
+    class Base {
+      @virtual int x;
+    }
+    class Derived extends Base {
+      int x;
+
+      // Expose the hidden storage slot:
+      int get superX => super.x;
+      set superX(int v) { super.x = v; }
+    }
+    ```
+
 * Breaking change - infer list and map literals from the context type as well as
     their values, consistent with generic methods and instance creation
     (SDK issue [27151](https://github.com/dart-lang/sdk/issues/27151)).
 
     ```dart
     import 'dart:async';
     main() async {
       var b = new Future<B>.value(new B());
       var c = new Future<C>.value(new C());
       var/*infer List<Future<A>>*/ list = [b, c];
@@ skipping 1202 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.