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

pkg/compiler/lib/src/tracer.dart

 // Copyright (c) 2014, 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 tracer;
 
 import '../compiler.dart' as api;
 import 'dart:async' show EventSink;
 import 'ssa/ssa.dart' as ssa;
 import 'ssa/ssa_tracer.dart' show HTracer;
@@ skipping 99 matching lines @@
   }
 
   void add(String string) {
     output.add(string);
   }
 
   void addIndent() {
     add(_ind.indentation);
   }
 }