Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/commandline_options.dart

 // Copyright (c) 2015, 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 dart2js.cmdline.options;
 
 /// Commandline flags used in `dart2js.dart` and/or `apiimpl.dart`.
 class Flags {
   static const String allowMockCompilation = '--allow-mock-compilation';
   static const String allowNativeExtensions = '--allow-native-extensions';
@@ skipping 17 matching lines @@
       '--experimental-track-allocations';
   static const String experimentalAllocationsPath =
       '--experimental-allocations-path';
   static const String fastStartup = '--fast-startup';
   static const String fatalWarnings = '--fatal-warnings';
   static const String generateCodeWithCompileTimeErrors =
       '--generate-code-with-compile-time-errors';
   // Temporary flag to also integrate Kernel into global type inference.
   // TODO(efortuna): Remove when fully implemented and simply use "useKernel".
   static const String kernelGlobalInference = '--kernel-global-inference';
-  // Read input from a .dill file rather than a .dart input (use only in
-  // conjunction with --use-kernel=true).
-  static const String loadFromDill = '--read-dill';
+
+  /// Enable the unified front end, loading from .dill files, and compilation
+  /// using the kernel representation.
+  /// See [CompilerOptions.previewDart2] for details.
+  // Note: this flag should only be changed in coordination with other teams,
+  // so the same flag is used everywhere.
+  static const String previewDart2 = '--preview-dart-2';
+
   static const String minify = '--minify';
   static const String noFrequencyBasedMinification =
       '--no-frequency-based-minification';
   static const String noSourceMaps = '--no-source-maps';
   static const String preserveComments = '--preserve-comments';
   static const String preserveUris = '--preserve-uris';
   static const String showPackageWarnings = '--show-package-warnings';
   static const String suppressHints = '--suppress-hints';
   static const String suppressWarnings = '--suppress-warnings';
   static const String terse = '--terse';
@@ skipping 43 matching lines @@
 }
 
 class Option {
   static const String showPackageWarnings =
       '${Flags.showPackageWarnings}|${Flags.showPackageWarnings}=.*';
 
   // Experimental options.
   static const String resolutionInput = '--resolution-input=.+';
   static const String bazelPaths = '--bazel-paths=.+';
 }