Deal with special cmd.exe environment variables that have keys that start with an equals sign.

runtime/bin/platform_impl.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.
 
 class _Platform {
   static int _numberOfProcessors() native "Platform_NumberOfProcessors";
   static String _pathSeparator() native "Platform_PathSeparator";
   static String _operatingSystem() native "Platform_OperatingSystem";
   static _localHostname() native "Platform_LocalHostname";
   static _environment() native "Platform_Environment";
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     }
   }
 
   static Map<String, String> get environment() {
     var env = _environment();
     if (env is OSError) {
       throw env;
     } else {
       var result = new Map();
       for (var str in env) {
+        // When running on Windows through cmd.exe there are strange
+        // environment variables that are used to record the current
+        // working directory for each drive and the exit code for the
+        // last command. As an example: '=A:=A:\subdir' records the
+        // current working directory on the 'A' drive.  In order to
+        // handle these correctly we search for a second occurrence of
+        // of '=' in the string if the first occurrence is at index 0.
         var equalsIndex = str.indexOf('=');
+        if (equalsIndex == 0) {
+          equalsIndex = str.indexOf('=', 1);
+        }
         assert(equalsIndex != -1);
         result[str.substring(0, equalsIndex)] = str.substring(equalsIndex + 1);
       }
       return result;
     }
   }
 }