Revert "Store library paths relative to a given application root folder."

pkg/kernel/lib/ast.dart

 // Copyright (c) 2016, 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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 139 matching lines @@
   Location _getLocationInEnclosingFile(int offset) {
     return parent?._getLocationInEnclosingFile(offset);
   }
 }
 
 // ------------------------------------------------------------------------
 //                      LIBRARIES and CLASSES
 // ------------------------------------------------------------------------
 
 class Library extends TreeNode implements Comparable<Library> {
-  /// An import path to this library.
+  /// An absolute import path to this library.
   ///
-  /// The [Uri] should have the `dart`, `package`, or `app` scheme.
-  ///
-  /// If the URI has the `app` scheme, it is relative to the application root.
+  /// The [Uri] should have the `dart`, `package`, or `file` scheme.
   Uri importUri;
 
   /// The uri of the source file this library was loaded from.
   String fileUri;
 
   /// 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
@@ skipping 3469 matching lines @@
 
   @override
   defaultTreeNode(TreeNode node) {
     if (node == child) {
       return replacement;
     } else {
       return node;
     }
   }
 }