Move StringImplementation from coreimpl to core as _StringImpl.

lib/core/string.dart

Index: lib/core/string.dart
diff --git a/lib/core/string.dart b/lib/core/string.dart
index 801e4151361c6e114492cc012c955e88875e9f31..ad4aefee7e1fca8b4e591dd6fed3f8de54c22e9d 100644
--- a/lib/core/string.dart
+++ b/lib/core/string.dart
@@ -10,7 +10,7 @@
  */
 interface String
     extends Comparable, Pattern, Sequence<String>
-    default StringImplementation {
+    default _StringImpl {
   /**
    * Allocates a new String for the specified [charCodes].
    */
@@ -131,3 +131,22 @@ 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);
+
+}