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

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
index cdc14c1c37a9279066a7ac4831f04da8572ddd81..5d9b9034b9b7122f621a16218d2de372d15da0c3 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -82,6 +82,35 @@ mixin(base, @rest mixins) => JS(
   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")');