Added VariableUse expression to tree IR.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index b1f03bfd730a7d0ba27a186869a9f6320274ff53..11f239b3e9fd4c9a491160ba47ab1cc4f31685f2 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -345,8 +345,12 @@ class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
   }
 
   @override
-  js.Expression visitVariable(tree_ir.Variable node) {
-    return new js.VariableUse(getVariableName(node));
+  js.Expression visitVariableUse(tree_ir.VariableUse node) {
+    return visitVariable(node.variable);
+  }
+
+  js.Expression visitVariable(tree_ir.Variable variable) {

[Kevin Millikin (Google), 2015/02/26 12:43:18]

Likewise, I'm not sure I'd call this visitVariable which suggests that I could
call it for any variable (it depends on the context, it doesn't do the right
thing for FunctionDefinition parameters).  Do we really have a js.VariableUse on
the left-hand side of an assignment?  Crazy.

[asgerf, 2015/02/27 12:05:19]

Renamed to buildVariableAccess.

+    return new js.VariableUse(getVariableName(variable));
   }
 
   @override