Introduce LibraryEntity

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

Index: pkg/compiler/lib/src/kernel/kernel_visitor.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_visitor.dart b/pkg/compiler/lib/src/kernel/kernel_visitor.dart
index c4ec66d10d3ee0f708f548146934ef5512206477..da51f989db9e3c7e1fc07491de9b8dff4734e433 100644
--- a/pkg/compiler/lib/src/kernel/kernel_visitor.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_visitor.dart
@@ -297,8 +297,7 @@ class KernelVisitor extends Object
 
   // TODO(ahe): Hack. Fix dart2js instead.
   ir.Name nameToIrName(Name name) {
-    assert(!name.isPrivate ||
-        name.library.implementation == currentElement.library.implementation);
+    assert(!name.isPrivate || name.library == currentElement.library);
     return kernel.irName(name.text, currentElement);
   }