[kernel] fix a typo

Change-Id: I60aaefd7c1166f862f0f207977ba10db6252d997
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/120924
Reviewed-by: Jens Johansen <[email protected]>
Commit-Queue: Devon Carew <[email protected]>

Author: devoncarew

pkg/kernel/test/class_hierarchy_test.dart

@@ -293,7 +293,7 @@ class H extends self::G implements self::C, self::A {}
 
   Class addClass(Class c) {
     if (_hierarchy != null) {
-      fail('The classs hierarchy has already been created.');
+      fail('The class hierarchy has already been created.');
     }
     library.addClass(c);
     return c;