Support for generic type variables and unresolved types added.

lib/compiler/implementation/dart_backend/placeholder.dart

Index: lib/compiler/implementation/dart_backend/placeholder.dart
diff --git a/lib/compiler/implementation/dart_backend/placeholder.dart b/lib/compiler/implementation/dart_backend/placeholder.dart
index 1b0545f7f639de10003907521893eae8044f6a00..cd79f698f4cc190f0d978501ecdc59467a0a7b32 100644
--- a/lib/compiler/implementation/dart_backend/placeholder.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 class Placeholder {
+  const Placeholder();
   abstract String rename(ConflictingRenamer renamer);
 }
 
@@ -26,3 +27,10 @@ class ElementPlaceholder extends Placeholder {
   String rename(ConflictingRenamer renamer) => renamer.renameElement(element);
   String toString() => 'element_placeholder[$element]';
 }
+
+class UnresolvedPlaceholder extends Placeholder {
+  const UnresolvedPlaceholder();
+  String rename(ConflictingRenamer renamer) =>
+      renamer.generateUniqueName('unresolved');
+  String toString() => 'unresolved_placeholder';
+}