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

tests/compiler/dart2js/unparser2_test.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.
 
 import "package:expect/expect.dart";
-import "package:compiler/src/scanner/scannerlib.dart";
-import "package:compiler/src/tree/tree.dart";
+import "package:compiler/implementation/scanner/scannerlib.dart";
+import "package:compiler/implementation/tree/tree.dart";
 
-import "package:compiler/src/dart2jslib.dart"
+import "package:compiler/implementation/dart2jslib.dart"
     show DiagnosticListener,
          Script;
 
-import "package:compiler/src/elements/elements.dart"
+import "package:compiler/implementation/elements/elements.dart"
     show CompilationUnitElement,
          LibraryElement;
 
-import "package:compiler/src/elements/modelx.dart"
+import "package:compiler/implementation/elements/modelx.dart"
     show CompilationUnitElementX,
          LibraryElementX;
 
 main() {
   testClassDef();
   testClass1Field();
   testClass2Fields();
   testClass1Field1Method();
   testClass1Field2Method();
   testClassDefTypeParam();
@@ skipping 51 matching lines @@
     messages.add(reason);
     throw reason;
   }
 
   void log(message) {
     messages.add(message);
   }
 
   noSuchMethod(Invocation invocation) => throw 'unsupported operation';
 }