Move StringImplementation from coreimpl to core as _StringImpl.

lib/core/strings.dart

Index: lib/core/strings.dart
diff --git a/lib/core/strings.dart b/lib/core/strings.dart
index e8cbcc024a4c4b4ebfc9111182a7c90008c465bf..86655e94ca32d38752f4d0363a8d29ddfc79b738 100644
--- a/lib/core/strings.dart
+++ b/lib/core/strings.dart
@@ -7,13 +7,13 @@ abstract class Strings {
    * Joins all the given strings to create a new string.
    */
   static String join(List<String> strings, String separator) {
-    return StringImplementation.join(strings, separator);
+    return _StringImpl.join(strings, separator);
   }
 
   /**
    * Concatenates all the given strings to create a new string.
    */
   static String concatAll(List<String> strings) {
-    return StringImplementation.concatAll(strings);
+    return _StringImpl.concatAll(strings);
   }
 }