Fix bugs found in checked mode.

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 365 matching lines @@
   final CompilationUnitElement compilationUnit;
 
   CompilationUnitOverrideElement(CompilationUnitElement compilationUnit,
                                  Element enclosing)
       : this.compilationUnit = compilationUnit,
         super(compilationUnit.name,
               ElementKind.COMPILATION_UNIT_OVERRIDE,
               enclosing);
 
   CompilationUnitElement asCompilationUnit() => compilationUnit;
+
+  CompilationUnitElement getCompilationUnit() => compilationUnit;
 }
 
 class LibraryElement extends ScopeContainerElement {
   CompilationUnitElement entryCompilationUnit;
   Link<CompilationUnitElement> compilationUnits =
       const EmptyLink<CompilationUnitElement>();
 
   Link<ScriptTag> tags = const EmptyLink<ScriptTag>();
   ScriptTag libraryTag;
   bool canUseNative = false;
@@ skipping 1029 matching lines @@
   Node parseNode(compiler) => cachedNode;
 
   String toString() => "${enclosingElement.toString()}.${name.slowToString()}";
 
   TypeVariableElement cloneTo(Element enclosing, DiagnosticListener listener) {
     TypeVariableElement result =
         new TypeVariableElement(name, enclosing, node, type, bound);
     return result;
   }
 }