Better mirrors support for mixins and private fields

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 1899f9683287243af62a52c22f6e2ba48bf4e8a3..629db3861208593319cb1cd1953a06c4d2f4e64d 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
@@ -64,12 +64,12 @@ mixin(base, @rest mixins) => JS(
 /// The Symbol for storing type arguments on a specialized generic type.
 final _mixins = JS('', 'Symbol("mixins")');
 
-getMixins(clazz) => JS('', '$clazz[$_mixins]');
+getMixins(clazz) => JS('', 'Object.hasOwnProperty.call(#, #) ? #[#] : null', clazz, _mixins, clazz, _mixins);
 
 @JSExportName('implements')
 final _implements = JS('', 'Symbol("implements")');
 
-getImplements(clazz) => JS('', '#[#]', clazz, _implements);
+getImplements(clazz) => JS('', 'Object.hasOwnProperty.call(#, #) ? #[#] : null', clazz, _implements, clazz, _implements);
 
 /// The Symbol for storing type arguments on a specialized generic type.
 final _typeArguments = JS('', 'Symbol("typeArguments")');