Refactor Parsing to remove compiler dependency

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';
@@ skipping 106 matching lines @@
   return node;
 }
 
 Node parseStatement(String text) =>
   parseBodyCode(text, (parser, tokens) => parser.parseStatement(tokens));
 
 Node parseFunction(String text, Compiler compiler) {
   ElementX element = parseUnit(text, compiler, compiler.mainApp).head;
   Expect.isNotNull(element);
   Expect.equals(ElementKind.FUNCTION, element.kind);
-  return element.parseNode(compiler.parsing);
+  return element.parseNode(compiler.parsingContext);
 }
 
 Node parseMember(String text, {DiagnosticReporter reporter}) {
   return parseBodyCode(
       text,
       (parser, tokens) => parser.parseMember(tokens),
       reporter: reporter);
 }
 
 class MockFile extends StringSourceFile {
   MockFile(text)
       : super.fromName('<string>', text);
 }
 
 var sourceCounter = 0;
 
 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);
   DiagnosticReporter reporter = compiler.reporter;
   ElementListener listener = new ElementListener(
-      compiler.parsing.getScannerOptionsFor(library),
+      compiler.parsingContext.getScannerOptionsFor(library),
       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);
 }