Store library paths relative to a given application root folder.

pkg/kernel/lib/ast.dart

Index: pkg/kernel/lib/ast.dart
diff --git a/pkg/kernel/lib/ast.dart b/pkg/kernel/lib/ast.dart
index 0cece08e0f1856084f0100c11751704957f7e3b4..862de37bd3bd6a27684f45a3f9bec651359d7aeb 100644
--- a/pkg/kernel/lib/ast.dart
+++ b/pkg/kernel/lib/ast.dart
@@ -157,9 +157,11 @@ abstract class TreeNode extends Node {
 // ------------------------------------------------------------------------
 
 class Library extends TreeNode implements Comparable<Library> {
-  /// An absolute import path to this library.
+  /// An import path to this library.
   ///
-  /// The [Uri] should have the `dart`, `package`, or `file` scheme.
+  /// The [Uri] should have the `dart`, `package`, or `app` scheme.
+  ///
+  /// If the URI has the `app` scheme, it is relative to the application root.
   Uri importUri;
 
   /// The uri of the source file this library was loaded from.