Fix accessing private members on extension classes.

tool/input_sdk/private/ddc_runtime/classes.dart

Index: tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/tool/input_sdk/private/ddc_runtime/classes.dart b/tool/input_sdk/private/ddc_runtime/classes.dart
index 56920e109f8d93a0dc9f23f5df363ff23d63c392..86923f83c33d6384d3c5f6a74cab5ebf5588b12b 100644
--- a/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -343,6 +343,9 @@ defineExtensionMembers(type, methodNames) => JS('', '''(() => {
 })()''');
 
 canonicalMember(obj, name) => JS('', '''(() => {
+  // Private names are symbols and are already canonical.
+  if (typeof name === 'symbol') return name;
+
   if ($obj != null && $obj[$_extensionType]) return $dartx[$name];
   // Check for certain names that we can't use in JS
   if ($name == 'constructor' || $name == 'prototype') {