Fix codegeneration for 'return'.

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 db1d80479a40fefbb0a8f3c8f00ad1e4dce6e5f2..faaa53527df76e109a00cecb9982e0353d4f75ff 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -282,9 +282,7 @@ class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
 
   @override
   void visitReturn(tree_ir.Return node) {
-    if (node.value != null) {
-      accumulator.add(new js.Return(visitExpression(node.value)));
-    }
+    accumulator.add(new js.Return(visitExpression(node.value)));
   }
 
   bool isNullLiteral(js.Expression exp) => exp is js.LiteralNull;