Support virtual fields in DDC without requiring the @virtual annotation.

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.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.
 
 /// This library defines the operations that define and manipulate Dart
 /// classes.  Included in this are:
 ///   - Generics
 ///   - Class metadata
 ///   - Extension methods
 ///
@@ skipping 64 matching lines @@
       }
       return s;
     }
   });
 
   // Save mixins for reflection
   Mixin[$_mixins] = $mixins;
   return Mixin;
 })()''');
 
+///
+/// Support for virtualizing fields on-demand.
+///
+final _virtualNames = JS('', '{}');
+
+virtualName(key) => JS(
+    '',
+    '''(() => {
+  return #[key] || (#[key] = Symbol(key));
+})()''',
+    _virtualNames,
+    _virtualNames);
+
+virtualize(object, keys) => JS(
+    '',
+    '''(() => {
+  for (var i = 0; i < keys.length; i += 2) {
+    let name = keys[i];
+    let symbol = keys[i + 1];
+    Object.defineProperty(object, name, {
+      configurable: true,
+      set: function(value) {
+        object[symbol] = value;
+        delete object[name];
+      }
+    });
+  }
+})()''');
+
 /// The Symbol for storing type arguments on a specialized generic type.
 final _mixins = JS('', 'Symbol("mixins")');
 
 getMixins(clazz) => JS('', 'Object.hasOwnProperty.call(#, #) ? #[#] : null',
     clazz, _mixins, clazz, _mixins);
 
 @JSExportName('implements')
 final _implements = JS('', 'Symbol("implements")');
 
 getImplements(clazz) => JS('', 'Object.hasOwnProperty.call(#, #) ? #[#] : null',
@@ skipping 539 matching lines @@
     '''(() => {
   let values = [];
   for (var i = 0; i < $names.length; i++) {
     let value = $const_(new $enumClass(i));
     values.push(value);
     Object.defineProperty($enumClass, $names[i],
         { value: value, configurable: true });
   }
   $enumClass.values = $constList(values, $enumClass);
 })()''');