Refactored treatment of closure variables in dart2js CPS.

pkg/compiler/lib/src/cps_ir/constant_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/constant_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/constant_propagation.dart b/pkg/compiler/lib/src/cps_ir/constant_propagation.dart
index 975a53180db7b95d9d15cc200b2b5e7cb0901305..c9adf089d055cb93124b59faaef4fa3db00dd64e 100644
--- a/pkg/compiler/lib/src/cps_ir/constant_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/constant_propagation.dart
@@ -308,7 +308,7 @@ class _ConstPropagationVisitor extends Visitor {
   }
 
   void visitFunctionDefinition(FunctionDefinition node) {
-    node.parameters.forEach(visitParameter);
+    node.parameters.forEach(visit);
     setReachable(node.body);
   }
 
@@ -586,6 +586,9 @@ class _ConstPropagationVisitor extends Visitor {
     setValue(node, _ConstnessLattice.NonConst);
   }
 
+  void visitClosureVariable(ClosureVariable node) {
+  }
+
   void visitParameter(Parameter node) {
     if (node.parent is FunctionDefinition) {
       // Functions may escape and thus their parameters must be initialized to