Check for null when getting the extension type.

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 373 matching lines @@
   return $name in sigObj;
 })()''');
 
 bool hasMethod(type, name) => _hasSigEntry(type, _methodSig, name);
 bool hasGetter(type, name) => _hasSigEntry(type, _getterSig, name);
 bool hasSetter(type, name) => _hasSigEntry(type, _setterSig, name);
 bool hasField(type, name) => _hasSigEntry(type, _fieldSig, name);
 
 final _extensionType = JS('', 'Symbol("extensionType")');
 
-getExtensionType(obj) => JS('', '#[#]', obj, _extensionType);
+getExtensionType(obj) =>
+    JS('', '# == null ? null : #[#]', obj, obj, _extensionType);
 
 final dartx = JS('', 'dartx');
 
 getExtensionSymbol(name) {
   var sym = JS('', 'dartx[#]', name);
   if (sym == null) {
     sym = JS('', 'Symbol("dartx." + #.toString())', name);
     JS('', 'dartx[#] = #', name, sym);
   }
   return sym;
@@ skipping 147 matching lines @@
 
 defineEnumValues(enumClass, names) {
   var values = [];
   for (var i = 0; i < JS('int', '#.length', names); i++) {
     var value = const_(JS('', 'new #.new(#)', enumClass, i));
     JS('', '#.push(#)', values, value);
     defineValue(enumClass, JS('', '#[#]', names, i), value);
   }
   JS('', '#.values = #', enumClass, constList(values, enumClass));
 }