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..25087ae3bb35e48afbc6ed211a417c65f350230c 100644
--- a/pkg/compiler/lib/src/commandline_options.dart
+++ b/pkg/compiler/lib/src/commandline_options.dart
@@ -35,9 +35,17 @@ 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.useKernel] for details.
+  static const String useKernel = '--use-kernel';
+
+  /// Only enable the kernel ssa builder, but continue using the old compiler
+  /// pipeline. Only used for testing the ssa builder.
+  // TODO(sigmund): delete.
+  static const String useKernelInSsa = '--use-kernel-in-ssa';
+
   static const String minify = '--minify';
   static const String noFrequencyBasedMinification =
       '--no-frequency-based-minification';
@@ -54,7 +62,6 @@ class Flags {
   static const String trustJSInteropTypeAnnotations =
       '--experimental-trust-js-interop-type-annotations';
   static const String useContentSecurityPolicy = '--csp';
-  static const String useKernel = '--use-kernel';
   static const String useMultiSourceInfo = '--use-multi-source-info';
   static const String useNewSourceInfo = '--use-new-source-info';
   static const String verbose = '--verbose';