dart2js cps: Compile some loops as 'for' loops.

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

Index: pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart b/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
index 79f018ba204d3015e358cc52f1275fe3e8c07523..91a5aac7d4c6367ae7fa746122cf67d44940a986 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/variable_merger.dart
@@ -208,10 +208,11 @@ class BlockGraphBuilder extends RecursiveVisitor {
     visitStatement(node.body); // visitContinue will add predecessors to join.
   }
 
-  visitWhileCondition(WhileCondition node) {
-    Block join = _jumpTarget[node.label] = newBlock();
-    join.predecessors.add(_currentBlock);
-    _currentBlock = join;
+  visitFor(For node) {
+    Block entry = _currentBlock;
+    _currentBlock = _jumpTarget[node.label] = newBlock();
+    node.updates.forEach(visitExpression);
+    joinFrom(entry, _currentBlock);
     visitExpression(node.condition);
     Block afterCondition = _currentBlock;
     branchFrom(afterCondition);