Convert String to a class.

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
diff --git a/sdk/lib/core/string.dart b/sdk/lib/core/string.dart
index ad4aefee7e1fca8b4e591dd6fed3f8de54c22e9d..f2d0e613203d238145d68ef8416cddb30d6c79b6 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -8,13 +8,11 @@
  * scalar character codes accessible through the [charCodeAt] or the
  * [charCodes] method.
  */
-interface String
-    extends Comparable, Pattern, Sequence<String>
-    default _StringImpl {
+abstract class String implements Comparable, Pattern, Sequence<String> {
   /**
    * Allocates a new String for the specified [charCodes].
    */
-  String.fromCharCodes(List<int> charCodes);
+  external factory String.fromCharCodes(List<int> charCodes);
 
   /**
    * Gets the character (as [String]) at the given [index].
@@ -131,22 +129,3 @@ interface String
    */
   String toUpperCase();
 }
-
-class _StringImpl {
-  /**
-   * Factory implementation of String.fromCharCodes:
-   * Allocates a new String for the specified [charCodes].
-   */
-  external factory String.fromCharCodes(List<int> charCodes);
-
-  /**
-   * Joins all the given strings to create a new string.
-   */
-  external static String join(List<String> strings, String separator);
-
-  /**
-   * Concatenates all the given strings to create a new string.
-   */
-  external static String concatAll(List<String> strings);
-
-}