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

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

Index: pkg/compiler/lib/src/dart_backend/backend.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend.dart b/pkg/compiler/lib/src/dart_backend/backend.dart
index f2c6c71bd43f08ed8e4a977ccd0cbcd6e6eee4ff..f84157b5b6f9eb6416f7b0b467df9cd888185f64 100644
--- a/pkg/compiler/lib/src/dart_backend/backend.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend.dart
@@ -168,11 +168,6 @@ class DartBackend extends Backend {
     applyCpsPass(new RedundantPhiEliminator());
     applyCpsPass(new ShrinkingReducer());
 
-    // Do not rewrite the IR after variable allocation.  Allocation
-    // makes decisions based on an approximation of IR variable live
-    // ranges that can be invalidated by transforming the IR.
-    new cps_ir.RegisterAllocator(context.internalError).visit(cpsDefinition);
-
     tree_builder.Builder builder =
         new tree_builder.Builder(context.internalError);
     tree_ir.ExecutableDefinition treeDefinition = builder.build(cpsDefinition);
@@ -188,7 +183,7 @@ class DartBackend extends Backend {
     }
 
     applyTreePass(new StatementRewriter());
-    applyTreePass(new CopyPropagator());
+    applyTreePass(new VariableMerger());
     applyTreePass(new LoopRewriter());
     applyTreePass(new LogicalRewriter());