[kernel] Fix the comment on Library.isExternal.

The comment was the result of a bad merge.

BUG=
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Review URL: https://chromereviews.googleplex.com/517957015 .

Author: asgerf

pkg/kernel/lib/ast.dart

@@ -149,8 +149,8 @@ class Library extends TreeNode implements Comparable<Library> {
   /// The uri of the source file this library was loaded from.
   String fileUri;
 
-  /// If false, the library object is a placeholder for a library that has
-  /// not been loaded yet.
+  /// If true, the library is part of another build unit and its contents
+  /// are only partially loaded.
   ///
   /// Classes of an external library are loaded at one of the [ClassLevel]s
   /// other than [ClassLevel.Body].  Members in an external library have no