A library element should know its compilation units.

dart/lib/compiler/implementation/elements/elements.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.
 
 #library('elements');
 
 #import('dart:uri');
 
 #import('../tree/tree.dart');
 #import('../scanner/scannerlib.dart');
@@ skipping 487 matching lines @@
         field.setter = accessor;
       }
       addToScope(field, listener);
     }
   }
 }
 
 class CompilationUnitElement extends ContainerElement {
   final Script script;
 
-  CompilationUnitElement(Script script, Element enclosing)
+  CompilationUnitElement(Script script, LibraryElement library)
     : this.script = script,
       super(new SourceString(script.name),
             ElementKind.COMPILATION_UNIT,
-            enclosing);
+            library) {
+    library.addCompilationUnit(this);
+  }
 
   void addMember(Element element, DiagnosticListener listener) {
     // Keep a list of top level members.
     super.addMember(element, listener);
     // Provide the member to the library to build scope.
     getLibrary().addMember(element, listener);
   }
 }
 
 class CompilationUnitOverrideElement extends Element {
@@ skipping 1267 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }