Don't use identical for null.

runtime/lib/integers_patch.dart

 // Copyright (c) 2012, 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.
 // Dart core library.
 
 // VM implementation of int.
 
 patch class int {
 
   static bool is64Bit() => 1 << 32 is _Smi;
@@ skipping 44 matching lines @@
 
   static int _native_parse(String str) native "Integer_parse";
 
   static int _throwFormatException(String source, int position) {
     throw new FormatException("", source, position);
   }
 
   /* patch */ static int parse(String source,
                                { int radix,
                                  int onError(String str) }) {
-    if (identical(source, null)) throw new ArgumentError(source);
+    if (source == null) throw new ArgumentError(source);
     if (radix == null) {
       int result;
       if (source.isNotEmpty) result = _parse(source);
       if (result == null) {
         if (onError == null) {
           throw new FormatException("", source);
         }
         return onError(source);
       }
       return result;
@@ skipping 46 matching lines @@
       int digit = digits[code - 0x30];
       if (digit >= radix) return onError(source);
       result = result * radix + digit;
       i++;
       if (i == end) break;
       code = source.codeUnitAt(i);
     } while (true);
     return negative ? -result : result;
   }
 }