fix spelling in core libraries

sdk/lib/core/string.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;
 
 /**
  * A sequence of characters.
  *
  * A string can be either single or multiline. Single line strings are
@@ skipping 395 matching lines @@
    * index:
    *
    *     string.contains('X', 1);                  // false
    *     string.contains(new RegExp(r'[A-Z]'), 1); // false
    *
    * [startIndex] must not be negative or greater than [length].
    */
   bool contains(Pattern other, [int startIndex = 0]);
 
   /**
-   * Returns a new string in which the first occurence of [from] in this string
+   * Returns a new string in which the first occurrence of [from] in this string
    * is replaced with [to], starting from [startIndex]:
    *
    *     '0.0001'.replaceFirst(new RegExp(r'0'), ''); // '.0001'
    *     '0.0001'.replaceFirst(new RegExp(r'0'), '7', 1); // '0.7001'
    */
   String replaceFirst(Pattern from, String to, [int startIndex = 0]);
 
   /**
-   * Replace the first occurence of [from] in this string.
+   * Replace the first occurrence of [from] in this string.
    *
    * Returns a new string, which is this string
    * except that the first match of [pattern], starting from [startIndex],
    * is replaced by the result of calling [replace] with the match object.
    *
    * The optional [startIndex] is by default set to 0. If provided, it must be
    * an integer in the range `[0 .. len]`, where `len` is this string's length.
    *
    * If the value returned by calling `replace` is not a [String], it
    * is converted to a `String` using its `toString` method, which must
@@ skipping 350 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }