dart2js cps: Remove dart2dart from cps pipeline and clean up.

pkg/compiler/lib/src/dart_backend/dart_backend.dart

Index: pkg/compiler/lib/src/dart_backend/dart_backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/dart_backend.dart b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
index 790daa03c32b8968d2a775258d9d15d78432bcd8..d8221ade60260b214c9da44ef9e56a77c1996ffb 100644
--- a/pkg/compiler/lib/src/dart_backend/dart_backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
@@ -11,23 +11,11 @@ import '../dart2jslib.dart';
 import '../library_loader.dart' show LoadedLibraries;
 import '../dart_types.dart';
 import '../tree/tree.dart';
-import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
-import '../cps_ir/cps_ir_nodes_sexpr.dart';
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
-import '../cps_ir/optimizers.dart';
-import '../cps_ir/optimizers.dart' as cps_opt;
-import '../tree_ir/tree_ir_builder.dart' as tree_builder;
-import '../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../util/util.dart';
 import '../mirror_renamer/mirror_renamer.dart';
-import '../tree_ir/optimization/optimization.dart';
-import '../tree_ir/optimization/optimization.dart' as tree_opt;
-import '../tree_ir/tree_ir_integrity.dart';
-import '../cps_ir/cps_ir_integrity.dart';
-import 'backend_ast_emitter.dart' as backend_ast_emitter;
-import 'backend_ast_nodes.dart' as backend_ast;
 import 'backend_ast_to_frontend_ast.dart' as backend2frontend;
 import '../../compiler.dart' show CompilerOutputProvider;