fix spelling in core libraries

sdk/lib/core/symbol.dart

Index: sdk/lib/core/symbol.dart
diff --git a/sdk/lib/core/symbol.dart b/sdk/lib/core/symbol.dart
index 44e9787b189949f95f8b343856204b774692c9e8..fc391417b603223a27fa33ecdbed71de4b0d4dde 100644
--- a/sdk/lib/core/symbol.dart
+++ b/sdk/lib/core/symbol.dart
@@ -23,7 +23,7 @@ abstract class Symbol {
    *   (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).