dart2js CPS: Clean up the S-Expression builder.

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

Index: pkg/compiler/lib/src/js_backend/codegen/task.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/task.dart b/pkg/compiler/lib/src/js_backend/codegen/task.dart
index 16093d3e59fba4c74dbcc207c1f4e0d7c049828f..bd52462019b4a03d1324b1ec01eed91cd7994e76 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/task.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/task.dart
@@ -151,8 +151,9 @@ class CpsFunctionCompiler implements FunctionCompiler {
         cps.Node node = nodeOrRef is cps.Reference
             ? nodeOrRef.definition
             : nodeOrRef;
-        var type = typePropagator.getType(node);
-        return type == null ? s : "$s:${formatTypeMask(type.type)}";
+        return node is cps.Variable && node.type != null
+            ? '$s:${formatTypeMask(node.type)}'
+            : s;
       }
       DEBUG_MODE = true;
       print(new SExpressionStringifier(printType).visit(cpsNode));