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

pkg/dart2js_incremental/lib/diff.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 trydart.poi.diff;
 
-import 'package:compiler/src/elements/elements.dart' show
+import 'package:compiler/implementation/elements/elements.dart' show
     AbstractFieldElement,
     ClassElement,
     CompilationUnitElement,
     Element,
     ElementCategory,
     FunctionElement,
     LibraryElement,
     ScopeContainerElement;
 
-import 'package:compiler/src/elements/modelx.dart' as modelx;
+import 'package:compiler/implementation/elements/modelx.dart' as modelx;
 
-import 'package:compiler/src/elements/modelx.dart' show
+import 'package:compiler/implementation/elements/modelx.dart' show
     DeclarationSite;
 
-import 'package:compiler/src/scanner/scannerlib.dart' show
+import 'package:compiler/implementation/scanner/scannerlib.dart' show
     EOF_TOKEN,
     ErrorToken,
     IDENTIFIER_TOKEN,
     KEYWORD_TOKEN,
     PartialClassElement,
     PartialElement,
     Token;
 
 class Difference {
   final DeclarationSite before;
@@ skipping 76 matching lines @@
       beforeToken = beforeToken.next;
       afterToken = afterToken.next;
       beforeKind = beforeToken.kind;
       afterKind = afterToken.kind;
     }
     return beforeKind != afterKind;
   }
   print("$before isn't a PartialElement");
   return true;
 }