Revert "Revert "Allow patch files to add top-level declarations to the patched library.""

lib/compiler/implementation/scanner/scanner_task.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.
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name() => 'Scanner';
 
   void scan(CompilationUnitElement compilationUnit) {
     measure(() {
@@ skipping 112 matching lines @@
     if (newLibrary) {
       compiler.withCurrentElement(library, () {
         scan(library);
         compiler.onLibraryLoaded(library, uri);
       });
     }
     return library;
   }
 
   void importLibrary(LibraryElement library, LibraryElement imported,
-                     ScriptTag tag, [CompilationUnitElement patch]) {
+                     ScriptTag tag, [CompilationUnitElement override]) {
     if (!imported.hasLibraryName()) {
       compiler.withCurrentElement(library, () {
         compiler.reportError(tag === null ? null : tag.argument,
                              'no #library tag found in ${imported.script.uri}');
       });
     }
     if (tag !== null && tag.prefix !== null) {
       SourceString prefix =
           new SourceString(tag.prefix.dartString.slowToString());
       Element e = library.find(prefix);
       if (e === null) {
-        e = new PrefixElement(prefix, library, tag.getBeginToken(), patch);
+        Element enclosing = library;
+        if (override !== null) {
+          enclosing = new CompilationUnitOverrideElement(override, library);
+        }
+        e = new PrefixElement(prefix, enclosing, tag.getBeginToken());
         library.define(e, compiler);
       }
       if (e.kind !== ElementKind.PREFIX) {
         compiler.withCurrentElement(e, () {
           compiler.reportWarning(new Identifier(e.position()),
                                  'duplicated definition');
         });
         compiler.reportError(tag.prefix, 'duplicate defintion');
       }
       PrefixElement prefixElement = e;
@@ skipping 48 matching lines @@
   static final int RESOURCE = 4;
 
   /** Next state. */
   static final List<int> NEXT =
       const <int>[NO_TAG_SEEN,
                   IMPORT, // Only one library tag is allowed.
                   IMPORT,
                   SOURCE,
                   RESOURCE];
 }