VM library tweaks to avoid megamorphic calls in dart2js

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index eb1a2a29553b9c55fd6f6371d07dde075b60e062..cdb94bea9506fef61da25ab5952f71d9239d5347 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -801,6 +801,7 @@ class _StringBase {
 
   // Convert single object to string.
   static String _interpolateSingle(Object o) {
+    if (o is String) return o;
     final s = o.toString();
     if (s is! String) {
       throw new ArgumentError(s);