cps-ir: Merge variables based on set-based liveness and graph coloring.

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 cd832feecfe76f847e102134f5ca14d8253fd856..143bcf53b465b0ee2fca1d3970c19bedfa6912af 100644
--- a/pkg/compiler/lib/src/dart_backend/dart_backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/dart_backend.dart
@@ -19,8 +19,7 @@ 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'
-    show LogicalRewriter, LoopRewriter, CopyPropagator, StatementRewriter;
+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';