Enqueue superclasses instead of supertypes.

pkg/compiler/lib/src/resolution/registry.dart

Index: pkg/compiler/lib/src/resolution/registry.dart
diff --git a/pkg/compiler/lib/src/resolution/registry.dart b/pkg/compiler/lib/src/resolution/registry.dart
index 64089c0681d03d461483dff61c449a7f16645320..47e5748766d608a6193526ac247775593e18da55 100644
--- a/pkg/compiler/lib/src/resolution/registry.dart
+++ b/pkg/compiler/lib/src/resolution/registry.dart
@@ -94,6 +94,8 @@ class EagerRegistry implements Registry {
     registerDependency(element);
     world.registerStaticUse(element);
   }
+
+  String toString() => 'EagerRegistry for ${mapping.analyzedElement}';
 }
 
 class ResolutionWorldImpact implements WorldImpact {
@@ -222,6 +224,8 @@ class ResolutionRegistry implements Registry {
 
   Backend get backend => compiler.backend;
 
+  String toString() => 'ResolutionRegistry for ${mapping.analyzedElement}';
+
   //////////////////////////////////////////////////////////////////////////////
   //  Node-to-Element mapping functionality.
   //////////////////////////////////////////////////////////////////////////////