Move dart2jslib parts into separate libraries.

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 fb3baa40721dc17474a79d859ee795d91945d126..44252e7414dd99043ee57c6a856e74a3224de7c9 100644
--- a/pkg/compiler/lib/src/js_backend/js_backend.dart
+++ b/pkg/compiler/lib/src/js_backend/js_backend.dart
@@ -18,6 +18,12 @@ import '../constants/values.dart';
 import '../dart2jslib.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
+import '../elements/visitor.dart' show
+    BaseElementVisitor;
+import '../enqueue.dart' show
+    Enqueuer,
+    ResolutionEnqueuer,
+    WorldImpact;
 import '../io/code_output.dart';
 import '../io/source_information.dart' show
     SourceInformationStrategy,
@@ -30,28 +36,33 @@ import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
 import '../js/js_source_mapping.dart' show
     JavaScriptSourceInformationStrategy;
-import '../js_emitter/js_emitter.dart'
-    show ClassBuilder, CodeEmitterTask, Emitter, MetadataCollector, Placeholder,
-        TokenFinalizer, USE_LAZY_EMITTER;
-
+import '../js/rewrite_async.dart';
+import '../js_emitter/js_emitter.dart' show
+    ClassBuilder,
+    CodeEmitterTask,
+    Emitter,
+    MetadataCollector,
+    Placeholder,
+    TokenFinalizer,
+    USE_LAZY_EMITTER;
 import '../library_loader.dart' show LibraryLoader, LoadedLibraries;
+import '../messages.dart' show MessageKind;
 import '../native/native.dart' as native;
+import '../resolution/resolution.dart' show
+    ResolutionRegistry,
+    TreeElements;
 import '../ssa/ssa.dart';
 import '../tree/tree.dart';
 import '../types/types.dart';
 import '../universe/universe.dart';
 import '../util/characters.dart';
 import '../util/util.dart';
+import '../world.dart' show
+    ClassWorld;
 
-import '../elements/visitor.dart' show
-    BaseElementVisitor;
-
-import '../js_backend/codegen/task.dart';
-import '../resolution/resolution.dart' show ResolutionRegistry;
-
+import 'codegen/task.dart';
 import 'constant_system_javascript.dart';
 import 'patch_resolver.dart';
-import '../js/rewrite_async.dart';
 
 part 'backend.dart';
 part 'checked_mode_helpers.dart';