Fix accessing private members on extension classes.

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 325 matching lines @@
   $defineMemoizedGetter(type, $_methodSig, function() {
     let sig = originalSigFn();
     for (let name of $methodNames) {
       sig[$getExtensionSymbol(name)] = sig[name];
     }
     return sig;
   });
 })()''');
 
 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') {
     $name = '+' + $name;
   }
   return $name;
 })()''');
 
 /// Sets the type of `obj` to be `type`
 setType(obj, type) => JS('', '''(() => {
   $obj.__proto__ = $type.prototype;
   // TODO(vsm): This should be set in registerExtension, but that is only
   // invoked on the generic type (e.g., JSArray<dynamic>, not JSArray<int>).
   $obj.__proto__[$_extensionType] = $type;
   return $obj;
 })()''');
 
 /// Sets the element type of a list literal.
 list(obj, elementType) =>
     JS('', '$setType($obj, ${getGenericClass(JSArray)}($elementType))');
 
 setBaseClass(derived, base) => JS('', '''(() => {
   // Link the extension to the type it's extending as a base class.
   $derived.prototype.__proto__ = $base.prototype;
 })()''');