Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

pkg/compiler/lib/src/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 source_file;
 
 import 'dart:math';
 import 'dart:convert' show UTF8;
 
 /**
@@ skipping 202 matching lines @@
 
   int get length => text.length;
   set length(int v) { }
 
   String slowText() => text;
 
   List<int> slowUtf8Bytes() => UTF8.encode(text);
 
   String slowSubstring(int start, int end) => text.substring(start, end);
 }