fix spelling in core libraries

sdk/lib/core/stacktrace.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.
 
 part of dart.core;
 
 /**
  * An interface implemented by all stack trace objects.
  *
  * A [StackTrace] is intended to convey information to the user about the call
@@ skipping 28 matching lines @@
    *
    * The getter achieves this without throwing, except on platforms that
    * have no other way to get a stack trace.
    */
   external static StackTrace get current;
 
   /**
    * Returns a [String] representation of the stack trace.
    *
    * The string represents the full stack trace starting from
-   * the point where a throw ocurred to the top of the current call sequence.
+   * the point where a throw occurred to the top of the current call sequence.
    *
    * The exact format of the string representation is not final.
    */
   String toString();
 }
 
 class _StringStackTrace implements StackTrace {
   final String _stackTrace;
   _StringStackTrace(this._stackTrace);
   String toString() => _stackTrace;
 }