fix #577 - correctly toString ES Symbols

tool/input_sdk/private/ddc_runtime/operations.dart

Index: tool/input_sdk/private/ddc_runtime/operations.dart
diff --git a/tool/input_sdk/private/ddc_runtime/operations.dart b/tool/input_sdk/private/ddc_runtime/operations.dart
index b9a324d7792a0b58e0599dda5fb1730680ddb3cc..3d7ea835c0a9cb9cd92e39e4faf023c6b642e8e6 100644
--- a/tool/input_sdk/private/ddc_runtime/operations.dart
+++ b/tool/input_sdk/private/ddc_runtime/operations.dart
@@ -459,14 +459,18 @@ hashCode(obj) {
 }
 
 @JSExportName('toString')
-_toString(obj) {
+String _toString(obj) {
   if (obj == null) return "null";
 
   var extension = getExtensionType(obj);
   if (extension != null) {
-    return JS('', '#[dartx.toString]()', obj);
+    return JS('String', '#[dartx.toString]()', obj);
   }
-  return JS('', '"" + #', obj);
+  // TODO(jmesserly): restore this faster path once ES Symbol is treated as
+  // an extension type (and thus hits the above code path).
+  // See https://github.com/dart-lang/dev_compiler/issues/578.
+  // return JS('', '"" + #', obj);
+  return JS('String', '#.toString()', obj);
 }
 
 // TODO(jmesserly): is the argument type verified statically?