Move StringImplementation from coreimpl to core as _StringImpl.

lib/core/string_buffer.dart

Index: lib/core/string_buffer.dart
diff --git a/lib/core/string_buffer.dart b/lib/core/string_buffer.dart
index 7530cdb8daba47bea6c48aefba43d14703b380b5..0561d83c217eaca346d937d0aab0d5aa038d3a44 100644
--- a/lib/core/string_buffer.dart
+++ b/lib/core/string_buffer.dart
@@ -117,7 +117,7 @@ class _StringBufferImpl implements StringBuffer {
   String toString() {
     if (_buffer.length === 0) return "";
     if (_buffer.length === 1) return _buffer[0];
-    String result = StringImplementation.concatAll(_buffer);
+    String result = _StringImpl.concatAll(_buffer);
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no