dart2dart: Bugfix in copy propagator.

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

Index: pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
index 0913a117172b7e3ea4c3bf6f168d5cf115606418..2670be74d251fcc79306b466bdace5632e591ad6 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/loop_rewriter.dart
@@ -43,13 +43,13 @@ class LoopRewriter extends RecursiveVisitor with PassMixin {
 
   Statement visitAssign(Assign node) {
     // Clean up redundant assignments left behind in the previous phase.
-    Expression def = node.definition;
-    if (def is VariableUse && node.variable == def.variable) {
+    Expression value = node.value;
+    if (value is VariableUse && node.variable == value.variable) {
       --node.variable.readCount;
       --node.variable.writeCount;
       return visitStatement(node.next);
     }
-    visitExpression(node.definition);
+    visitExpression(node.value);
     node.next = visitStatement(node.next);
     return node;
   }