Add a flag to control the backend.

lib/compiler/implementation/compiler.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.
 
 class WorkItem {
   final Element element;
   TreeElements resolutionTree;
   bool allowSpeculativeOptimization = true;
   List<HTypeGuard> guards = const <HTypeGuard>[];
 
@@ skipping 55 matching lines @@
     return generator.generateMethod(work, graph);
   }
 
   void processNativeClasses(libraries) {
     native.processNativeClasses(emitter, libraries);
   }
 
   void assembleProgram() => emitter.assembleProgram();
 }
 
+class DartBackend extends Backend {
+  DartBackend(Compiler compiler) : super(compiler);
+}
+
 class Compiler implements DiagnosticListener {
   final Map<String, LibraryElement> libraries;
   int nextFreeClassId = 0;
   World world;
   String assembledCode;
   Namer namer;
   Types types;
-  bool enableTypeAssertions = false;
+  final bool enableTypeAssertions;
 
   final Tracer tracer;
 
   CompilerTask measuredTask;
   Element _currentElement;
   LibraryElement coreLibrary;
   LibraryElement coreImplLibrary;
   LibraryElement isolateLibrary;
   LibraryElement jsHelperLibrary;
   LibraryElement interceptorsLibrary;
@@ skipping 36 matching lines @@
   static final SourceString MAIN = const SourceString('main');
   static final SourceString NO_SUCH_METHOD = const SourceString('noSuchMethod');
   static final SourceString NO_SUCH_METHOD_EXCEPTION =
       const SourceString('NoSuchMethodException');
   static final SourceString START_ROOT_ISOLATE =
       const SourceString('startRootIsolate');
   bool enabledNoSuchMethod = false;
 
   Stopwatch codegenProgress;
 
-  Compiler([this.tracer = const Tracer()])
+  Compiler([this.tracer = const Tracer(),
+            this.enableTypeAssertions = false,
+            bool emitJavascript = true])
       : libraries = new Map<String, LibraryElement>(),
         world = new World(),
         codegenProgress = new Stopwatch.start() {
     namer = new Namer(this);
     constantHandler = new ConstantHandler(this);
     scanner = new ScannerTask(this);
     dietParser = new DietParserTask(this);
     parser = new ParserTask(this);
     validator = new TreeValidatorTask(this);
     resolver = new ResolverTask(this);
     checker = new TypeCheckerTask(this);
-    backend = new JavaScriptBackend(this);
+    backend = emitJavascript ?
+        new JavaScriptBackend(this) : new DartBackend(this);
     enqueuer = new EnqueueTask(this);
     tasks = [scanner, dietParser, parser, resolver, checker,
              constantHandler, enqueuer];
   }
 
   Universe get codegenWorld() => enqueuer.codegen.universe;
 
   int getNextFreeClassId() => nextFreeClassId++;
 
   void ensure(bool condition) {
@@ skipping 429 matching lines @@
 When compiling the above method, the compiler crashed. It is not
 possible to tell if this is caused by a problem in your program or
 not. Regardless, the compiler should not crash.
 
 The Dart team would greatly appreciate if you would take a moment to
 report this problem at http://dartbug.com/new.
 
 Please copy and paste the error and stack trace that you see here into
 the bug report, include your OS and the Dart SDK build number.
 ''';