Hide internal details of VM string implementation.

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 07f3818027962bca0c14cbef3775c71a07cb7a1c..b59e06a5e61824e246fb74c6f6998f79af6881a5 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -4,14 +4,14 @@
 
 patch class StringImplementation {
   /* patch */ static String _fromCharCodes(List<int> charCodes) {
-    return StringBase.createFromCharCodes(charCodes);
+    return _StringBase.createFromCharCodes(charCodes);
   }
 
   /* patch */ static String join(List<String> strings, String separator) {
-    return StringBase.join(strings, separator);
+    return _StringBase.join(strings, separator);
   }
 
   /* patch */ static String concatAll(List<String> strings) {
-    return StringBase.concatAll(strings);
+    return _StringBase.concatAll(strings);
   }
 }