Convert String to a class.

sdk/lib/core/strings.dart

Index: sdk/lib/core/strings.dart
diff --git a/sdk/lib/core/strings.dart b/sdk/lib/core/strings.dart
index 86655e94ca32d38752f4d0363a8d29ddfc79b738..5029b46d4badd6f8f560b4dc32fa1410078ec189 100644
--- a/sdk/lib/core/strings.dart
+++ b/sdk/lib/core/strings.dart
@@ -6,14 +6,10 @@ abstract class Strings {
   /**
    * Joins all the given strings to create a new string.
    */
-  static String join(List<String> strings, String separator) {
-    return _StringImpl.join(strings, separator);
-  }
+  external static String join(List<String> strings, String separator);
 
   /**
    * Concatenates all the given strings to create a new string.
    */
-  static String concatAll(List<String> strings) {
-    return _StringImpl.concatAll(strings);
-  }
+  external static String concatAll(List<String> strings);
 }