Remove dart backend

test/dart_codegen/expect/core/string_buffer.dart

-part of dart.core;
- class StringBuffer implements StringSink {external StringBuffer([Object content = ""]);
- external int get length;
- bool get isEmpty => length == 0;
- bool get isNotEmpty => !isEmpty;
- external void write(Object obj);
- external void writeCharCode(int charCode);
- void writeAll(Iterable objects, [String separator = ""]) {
-  Iterator iterator = objects.iterator;
-   if (!iterator.moveNext()) return; if (separator.isEmpty) {
-    do {
-      write(iterator.current);
-      }
-     while (iterator.moveNext());}
-   else {
-    write(iterator.current);
-     while (iterator.moveNext()) {
-      write(separator);
-       write(iterator.current);
-      }
-    }
-  }
- void writeln([Object obj = ""]) {
-  write(obj);
-   write("\n");
-  }
- external void clear();
- external String toString();
-}