coverage: increase coverage of js_codegen

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 6f135f0d21cad9d566835518470bb9e35b167124..f9ceddb85afe64a7d9c2b4c237f312e33e7d2a4e 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -1624,12 +1624,12 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     if (entries.isEmpty) return js.call('dart.map()');
 
     // Use JS object literal notation if possible, otherwise use an array.
-    if (entries.every((e) => e.key is SimpleStringLiteral)) {
+    // We could do this any time all keys are non-nullable String type.
+    // For now, support StringLiteral as the common non-nullable String case.
+    if (entries.every((e) => e.key is StringLiteral)) {
       var props = [];
       for (var e in entries) {
-        var key = (e.key as SimpleStringLiteral).value;
-        var value = _visit(e.value);
-        props.add(new JS.Property(js.escapedString(key), value));
+        props.add(new JS.Property(_visit(e.key), _visit(e.value)));
       }
       mapArguments = new JS.ObjectInitializer(props);
     } else {