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

pkg/compiler/lib/src/tree_ir/optimization/optimization.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart b/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
index fc7aca15e8b84d428c45ae20e38ad6c01c16b230..1b66614b563f6fe0cd6554521b177b1594227747 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/optimization.dart
@@ -3,8 +3,10 @@ library tree_ir.optimization;
 import '../tree_ir_nodes.dart';
 import '../../elements/elements.dart';
 import '../../constants/values.dart' as values;
+import 'variable_merger.dart';
+
+export 'variable_merger.dart' show VariableMerger;
 
-part 'copy_propagator.dart';
 part 'logical_rewriter.dart';
 part 'loop_rewriter.dart';
 part 'statement_rewriter.dart';
@@ -36,4 +38,5 @@ abstract class PassMixin implements Pass {
     if (root.isAbstract) return;
     rewriteExecutableDefinition(root);
   }
-}
+}
+