pkg/compiler: fix imports

pkg/compiler/lib/src/io/source_file.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dart2js.io.source_file;
 
+import 'dart:convert' show UTF8;
 import 'dart:math';
-import 'dart:convert' show UTF8;
 import 'dart:typed_data' show Uint8List;
 
 import 'line_column_provider.dart';
 
 /**
  * Represents a file of source code. The content can be either a [String] or
  * a UTF-8 encoded [List<int>] of bytes.
  */
 abstract class SourceFile implements LineColumnProvider {
   /// The absolute URI of the source file.
@@ skipping 266 matching lines @@
   set length(int v) {}
 
   String slowText() => text;
 
   List<int> slowUtf8ZeroTerminatedBytes() {
     return _zeroTerminateIfNecessary(UTF8.encode(text));
   }
 
   String slowSubstring(int start, int end) => text.substring(start, end);
 }