Extract ResolutionWorldBuilderBase from ElementResolutionWorldBuilder

pkg/compiler/lib/src/kernel/world_builder.dart

Index: pkg/compiler/lib/src/kernel/world_builder.dart
diff --git a/pkg/compiler/lib/src/kernel/world_builder.dart b/pkg/compiler/lib/src/kernel/world_builder.dart
index c1dc21e2f2746440109d6f958c12c293644f2953..7713ae1c0b15a3ea1d169262648281156359850c 100644
--- a/pkg/compiler/lib/src/kernel/world_builder.dart
+++ b/pkg/compiler/lib/src/kernel/world_builder.dart
@@ -159,7 +159,8 @@ class KernelWorldBuilder extends KernelElementAdapterMixin {
         classEnv = _libraryEnvs[library.libraryIndex].lookupClass(node.name);
       }
       _classEnvs.add(classEnv);
-      return new KClass(library, _classMap.length, node.name);
+      return new KClass(library, _classMap.length, node.name,
+          isAbstract: node.isAbstract);
     });
   }
 
@@ -602,6 +603,11 @@ class KernelElementEnvironment implements ElementEnvironment {
   }
 
   @override
+  void forEachSupertype(ClassEntity cls, void f(InterfaceType supertype)) {
+    throw new UnimplementedError('KernelElementEnvironment.forEachSupertype');
+  }
+
+  @override
   void forEachMixin(ClassEntity cls, void f(ClassEntity mixin)) {
     throw new UnimplementedError('KernelElementEnvironment.forEachMixin');
   }