fix spelling in core libraries

sdk/lib/core/symbol.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;
 
 /// Opaque name used by mirrors, invocations and [Function.apply].
 abstract class Symbol {
   /**
    * Constructs a new Symbol.
    *
    * The name must be a valid public Dart member name,
    * public constructor name, or library name, optionally qualified.
    *
    * A qualified name is a valid name preceded by a public identifier name
    * and a '`.`', e.g., `foo.bar.baz=` is a qualified version of `baz=`.
    * That means that the content of the [name] String must be either
    *
    * * a valid public Dart identifier
    *   (that is, an identifier not starting with "`_`"),
    * * such an identifier followed by "=" (a setter name),
    * * the name of a declarable operator
    *   (one of "`+`", "`-`", "`*`", "`/`", "`%`", "`~/`", "`&`", "`|`",
    *   "`^`", "`~`", "`<<`", "`>>`", "`<`", "`<=`", "`>`", "`>=`", "`==`",
    *   "`[]`", "`[]=`", or "`unary-`"),
-   * * any of the above preceeded by any number of qualifiers,
+   * * any of the above preceded by any number of qualifiers,
    *   where a qualifier is a non-private identifier followed by '`.`',
    * * or the empty string (the default name of a library with no library
    *   name declaration).
    *
    * The following text is non-normative:
    *
    * Creating non-const Symbol instances may result in larger output.  If
    * possible, use [MirrorsUsed] in "dart:mirrors" to specify which names might
    * be passed to this constructor.
    */
   const factory Symbol(String name) = internal.Symbol;
 
   /**
    * Returns a hash code compatible with [operator==].
    *
    * Equal symbols have the same hash code.
    */
   int get hashCode;
 
   /**
    * Symbols are equal to other symbols that correspond to the same member name.
    *
    * Qualified member names, like `#foo.bar` are equal only if they have the
    * same identifiers before the same final member name.
    */
   bool operator ==(other);
 }