Side effect characterization of String.fromCharCode

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index 2490448ff809e2ea8ac1f62bb8a50d24fa63d478..20ac448df5fd9988f1c7e5cc0d168ee7e502a7ee 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -1017,13 +1017,15 @@ class Primitives {
   static String stringFromCharCode(charCode) {
     if (0 <= charCode) {
       if (charCode <= 0xffff) {
-        return JS('String', 'String.fromCharCode(#)', charCode);
+        return JS('returns:String;effects:none;depends:none',
+                  'String.fromCharCode(#)', charCode);
       }
       if (charCode <= 0x10ffff) {
         var bits = charCode - 0x10000;
         var low = 0xDC00 | (bits & 0x3ff);
         var high = 0xD800 | (bits >> 10);
-        return  JS('String', 'String.fromCharCode(#, #)', high, low);
+        return JS('returns:String;effects:none;depends:none',
+                  'String.fromCharCode(#, #)', high, low);
       }
     }
     throw new RangeError.range(charCode, 0, 0x10ffff);