A library element should know its compilation units.

dart/lib/compiler/implementation/patch_parser.dart

 // Copyright (c) 2012, 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.
 
 /**
  * TODO(johnniwinther): The terminology and invariants described below will not
  * hold until CL 10905305 has been committed.
  *
  * This library contains the infrastructure to parse and integrate patch files.
  *
@@ skipping 117 matching lines @@
 
   /**
    * Scans a library patch file, applies the method patches and
    * injections to the library, and returns a list of class
    * patches.
    */
   void patchLibrary(Uri patchUri, LibraryElement library) {
     leg.Script script = compiler.readScript(patchUri, null);
     CompilationUnitElement compilationUnit =
         new CompilationUnitElement(script, library);
-    library.addCompilationUnit(compilationUnit);
     LinkBuilder<tree.LibraryTag> imports = new LinkBuilder<tree.LibraryTag>();
     compiler.withCurrentElement(compilationUnit, () {
       // This patches the elements of the patch library into [library].
       // Injected elements are added directly under the compilation unit.
       // Patch elements are stored on the patched functions or classes.
       scanLibraryElements(compilationUnit, imports);
     });
     // After scanning declarations, we handle the import tags in the patch.
     // TODO(lrn): These imports end up in the original library and are in
     // scope for the original methods too. This should be fixed.
@@ skipping 253 matching lines @@
     super.addMember(element);
   }
 }
 
 // TODO(ahe): Get rid of this class.
 class PatchMetadataAnnotation extends MetadataAnnotation {
   final leg.Constant value = null;
 
   PatchMetadataAnnotation() : super(STATE_DONE);
 }