Add a flag to control the backend.

lib/compiler/implementation/apiimpl.dart

 // Copyright (c) 2012, 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('leg_apiimpl');
 
 #import('dart:uri');
 
 #import('../compiler.dart', prefix: 'api');
 #import('leg.dart', prefix: 'leg');
 #import('tree/tree.dart', prefix: 'tree');
 #import('elements/elements.dart', prefix: 'elements');
 #import('ssa/tracer.dart', prefix: 'ssa');
 #import('library_map.dart');
 #import('source_file.dart');
 
 class Compiler extends leg.Compiler {
   api.ReadUriFromString provider;
   api.DiagnosticHandler handler;
   Uri libraryRoot;
   Uri packageRoot;
   List<String> options;
   bool mockableLibraryUsed = false;
 
   Compiler(this.provider, this.handler, this.libraryRoot, this.options)
     : super(tracer: new ssa.HTracer()) {
-    enableTypeAssertions = options.indexOf('--enable-checked-mode') !== -1;
+    enableTypeAssertions = options.indexOf('--enable-checked-mode') != -1;
+    backend = options.indexOf('--2dart') != -1 ?
+        new leg.DartBackend(this) : new leg.JavaScriptBackend(this);
   }
 
   elements.LibraryElement scanBuiltinLibrary(String path) {
     Uri uri = libraryRoot.resolve(DART2JS_LIBRARY_MAP[path]);
     elements.LibraryElement library = scanner.loadLibrary(uri, null);
     return library;
   }
 
   void log(message) {
     handler(null, null, null, message, false);
@@ skipping 59 matching lines @@
     } else {
       handler(span.uri, span.begin, span.end, message, fatal);
     }
   }
 
   bool get isMockCompilation() {
     return mockableLibraryUsed
       && (options.indexOf('--allow-mock-compilation') !== -1);
   }
 }