Begin refactoring compiler out of diet parser and scanner

pkg/compiler/lib/src/id_generator.dart

Index: pkg/compiler/lib/src/id_generator.dart
diff --git a/tests/compiler/dart2js_extra/deferred_custom_loader_lib.dart b/pkg/compiler/lib/src/id_generator.dart
similarity index 52%
copy from tests/compiler/dart2js_extra/deferred_custom_loader_lib.dart
copy to pkg/compiler/lib/src/id_generator.dart
index e8636078fd29a22beb9a84b16232df299e6f114d..ce393f9b63b4efc378429e6fcaee041323d81285 100644
--- a/tests/compiler/dart2js_extra/deferred_custom_loader_lib.dart
+++ b/pkg/compiler/lib/src/id_generator.dart
@@ -2,4 +2,10 @@
 // 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.
 
-foo() => 499;
+/// A generator of globally unique identifiers.
+abstract class IdGenerator {
+  /// Returns the next free identifier.
+  ///
+  /// This identifier is guaranteed to be unique.
+  int getNextFreeId();
+}