fix spelling in core libraries

sdk/lib/core/num.dart

Index: sdk/lib/core/num.dart
diff --git a/sdk/lib/core/num.dart b/sdk/lib/core/num.dart
index 3d59c752e9fc006941f80c585bb3be651d2b193b..15b892d9c382ca0e3775c8342383277e1659f902 100644
--- a/sdk/lib/core/num.dart
+++ b/sdk/lib/core/num.dart
@@ -61,7 +61,7 @@ abstract class num implements Comparable<num> {
    * Returns a negative number if `this` is less than `other`, zero if they are
    * equal, and a positive number if `this` is greater than `other`.
    *
-   * The orderding represented by this method is a total ordering of [num]
+   * The ordering represented by this method is a total ordering of [num]
    * values. All distinct doubles are non-equal, as are all distinct integers,
    * but integers are equal to doubles if they have the same numerical
    * value.