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

runtime/bin/process_impl.dart

Index: runtime/bin/process_impl.dart
diff --git a/runtime/bin/process_impl.dart b/runtime/bin/process_impl.dart
index e2ba3e654576f856d059f2828b03165d98f590d5..00ddeae67a024d4a29a001e4cbe16920e12e206a 100644
--- a/runtime/bin/process_impl.dart
+++ b/runtime/bin/process_impl.dart
@@ -64,10 +64,6 @@ class _Process extends Process {
           throw new IllegalArgumentException(
               "Environment key or value is not a string: ($key, $value)");
         }
-        if (key.contains('=')) {
-          throw new IllegalArgumentException(
-              "Environment keys may not contain '=': $key");
-        }
         _environment.add('$key=$value');
       });
     }