fix spelling in core libraries

sdk/lib/core/invocation.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.
 
 part of dart.core;
 
 /**
  * Representation of the invocation of a member on an object.
  *
  * This is the type of objects passed to [Object.noSuchMethod] when
@@ skipping 24 matching lines @@
 
   /**
    * Whether the invocation was a getter call.
    * If so, both types of arguments is empty.
    */
   bool get isGetter;
 
   /**
    * Whether the invocation was a setter call.
    *
-   * If so, [arguments] has exactly one positonal argument,
+   * If so, [arguments] has exactly one positional argument,
    * and [namedArguments] is empty.
    */
   bool get isSetter;
 
   /** Whether the invocation was a getter or a setter call. */
   bool get isAccessor => isGetter || isSetter;
 }