fix spelling in core libraries

sdk/lib/core/pattern.dart

 // Copyright (c) 2011, 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 for basic searches within strings.
  */
 abstract class Pattern {
   // NOTE: When using "start" index from the language library, call
-  // without an argument if start is zero. This allows backwards compatiblity
+  // without an argument if start is zero. This allows backwards compatibility
   // with implementations of the older interface that didn't have the start
   // index argument.
   /**
    * Match this pattern against the string repeatedly.
    *
    * If [start] is provided, matching will start at that index.
    *
    * The returned iterable lazily computes all the non-overlapping matches
    * of the pattern on the string, ordered by start index.
    * If a user only requests the first
@@ skipping 36 matching lines @@
  *       String match = m.group(0);
  *       print(match);
  *     }
  *
  * The output of the example is:
  *
  *     Parse
  *     my
  *     string
  *
- * Some patterns, regular expressions in particular, may record subtrings
+ * Some patterns, regular expressions in particular, may record substrings
  * that were part of the matching. These are called _groups_ in the Match
  * object. Some patterns may never have any groups, and their matches always
  * have zero [groupCount].
  */
 abstract class Match {
   /**
    * Returns the index in the string where the match starts.
    */
   int get start;
 
@@ skipping 42 matching lines @@
   /**
    * The string on which this match was computed.
    */
   String get input;
 
   /**
    * The pattern used to search in [input].
    */
   Pattern get pattern;
 }