Fix function subtyping bug.

sdk/lib/_internal/lib/js_rti.dart

 // Copyright (c) 2013, 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 part contains helpers for supporting runtime type information.
  *
  * The helper use a mixture of Dart and JavaScript objects. To indicate which is
  * used where we adopt the scheme of using explicit type annotation for Dart
  * objects and 'var' or omitted return type for JavaScript objects.
@@ skipping 657 matching lines @@
     int sPos = 0;
     int tPos = pos;
     // Check the remaining parameters of [t] with the first optional parameters
     // of [s].
     for (; tPos < tParametersLen ; sPos++, tPos++) {
       if (!isAssignable(getIndex(sOptionalParameterTypes, sPos),
                         getIndex(tParameterTypes, tPos))) {
         return false;
       }
     }
-    sPos = 0;
-    // Check the optional parameters of [t] with the remaing optional parameters
-    // of [s]:
+    tPos = 0;
+    // Check the optional parameters of [t] with the remaining optional
+    // parameters of [s]:
     for (; tPos < tOptionalParametersLen ; sPos++, tPos++) {
-      if (!isAssignable(getIndex(tOptionalParameterTypes, sPos),
+      if (!isAssignable(getIndex(sOptionalParameterTypes, sPos),
                         getIndex(tOptionalParameterTypes, tPos))) {
         return false;
       }
     }
   }
 
   var sNamedParameters =
       getField(s, '${JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG()}');
   var tNamedParameters =
       getField(t, '${JS_FUNCTION_TYPE_NAMED_PARAMETERS_TAG()}');
@@ skipping 69 matching lines @@
  * and [t] are Dart values.
  */
 bool isIdentical(var s, var t) => JS('bool', '# === #', s, t);
 
 /**
  * Returns [:true:] if the JavaScript values [s] and [t] are not identical. We
  * use this helper to avoid generating code under the invalid assumption that
  * [s] and [t] are Dart values.
  */
 bool isNotIdentical(var s, var t) => JS('bool', '# !== #', s, t);