Support deserialized compilation of the empty program.

pkg/compiler/lib/src/js_backend/js_backend.dart

Index: pkg/compiler/lib/src/js_backend/js_backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/js_backend.dart b/pkg/compiler/lib/src/js_backend/js_backend.dart
index 8f1a807359d6f4eb68ea68e07a6cbd220cdadd3a..940c851f4ea6ee83e8ad4c2e4b290365fbd9c4ee 100644
--- a/pkg/compiler/lib/src/js_backend/js_backend.dart
+++ b/pkg/compiler/lib/src/js_backend/js_backend.dart
@@ -18,7 +18,7 @@ import '../common/codegen.dart' show CodegenImpact, CodegenWorkItem;
 import '../common/names.dart' show Identifiers, Names, Selectors, Uris;
 import '../common/registry.dart' show EagerRegistry, Registry;
 import '../common/resolution.dart'
-    show Feature, ListLiteralUse, MapLiteralUse, Resolution, ResolutionImpact;
+    show Feature, Frontend, ListLiteralUse, MapLiteralUse, Resolution, ResolutionImpact;
 import '../common/tasks.dart' show CompilerTask;
 import '../common/work.dart' show ItemCompilationContext;
 import '../compile_time_constants.dart';
@@ -32,6 +32,7 @@ import '../deferred_load.dart' show DeferredLoadTask;
 import '../diagnostics/invariant.dart' show DEBUG_MODE;
 import '../dump_info.dart' show DumpInfoTask;
 import '../elements/elements.dart';
+import '../elements/modelx.dart' show ConstructorBodyElementX;
 import '../elements/visitor.dart' show BaseElementVisitor;
 import '../enqueue.dart' show Enqueuer, ResolutionEnqueuer;
 import '../io/code_output.dart';
@@ -91,3 +92,4 @@ part 'namer_names.dart';
 part 'no_such_method_registry.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';
+