Begin refactoring compiler out of diet parser and scanner

tests/compiler/dart2js/parser_helper.dart

 // Copyright (c) 2016, 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 parser_helper;
 
 import "package:expect/expect.dart";
 
 import "package:compiler/src/elements/elements.dart";
+import 'package:compiler/src/id_generator.dart';
 import "package:compiler/src/tree/tree.dart";
 import "package:compiler/src/parser/element_listener.dart";
 import "package:compiler/src/parser/node_listener.dart";
 import "package:compiler/src/parser/parser.dart";
 import "package:compiler/src/parser/partial_parser.dart";
 import "package:compiler/src/scanner/string_scanner.dart";
 import "package:compiler/src/tokens/token.dart";
 import "package:compiler/src/tokens/token_constants.dart";
 import "package:compiler/src/io/source_file.dart";
 import "package:compiler/src/util/util.dart";
@@ skipping 123 matching lines @@
 Link<Element> parseUnit(String text, Compiler compiler,
                         LibraryElement library,
                         [void registerSource(Uri uri, String source)]) {
   Token tokens = scan(text);
   Uri uri = new Uri(scheme: "source", path: '${++sourceCounter}');
   if (registerSource != null) {
     registerSource(uri, text);
   }
   var script = new Script(uri, uri, new MockFile(text));
   var unit = new CompilationUnitElementX(script, library);
-  int id = 0;
   DiagnosticReporter reporter = compiler.reporter;
   ElementListener listener = new ElementListener(
       compiler.parsing.getScannerOptionsFor(library),
-      reporter, unit, () => id++);
+      reporter, unit, new IdGenerator());
   PartialParser parser = new PartialParser(listener, new MockParserOptions());
   reporter.withCurrentElement(unit, () => parser.parseUnit(tokens));
   return unit.localMembers;
 }
 
 NodeList fullParseUnit(String source, {DiagnosticReporter reporter}) {
   return parseBodyCode(
       source,
       (parser, tokens) => parser.parseUnit(tokens),
       reporter: reporter);
 }