Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/commandline_options.dart

Index: pkg/compiler/lib/src/commandline_options.dart
diff --git a/pkg/compiler/lib/src/commandline_options.dart b/pkg/compiler/lib/src/commandline_options.dart
index e1a646e26fb69b97b42b60f718a0b7d86338355c..b8a22076ae5c74ba2638518d79d39b1cd6819fec 100644
--- a/pkg/compiler/lib/src/commandline_options.dart
+++ b/pkg/compiler/lib/src/commandline_options.dart
@@ -35,9 +35,14 @@ class Flags {
   // 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';