fixes #259, clone ast before mutation

lib/src/compiler.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.
 
 /// Command line tool to run the checker on a Dart program.
 library dev_compiler.src.compiler;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:math' as math;
@@ skipping 110 matching lines @@
     }
 
     if (failureInLib) {
       _failure = true;
       if (!options.codegenOptions.forceCompile) return;
     }
 
     if (_jsGen != null) {
       var unit = units.first;
       var parts = units.skip(1).toList();
-
-      // TODO(jmesserly): this hack is to avoid compiling the same compilation
-      // unit to JS twice. We mutate the AST, so it's not safe to run more than
-      // once on the same unit.
-      if (unit.getProperty(_propertyName) == true) return;
-      unit.setProperty(_propertyName, true);
-
       _jsGen.generateLibrary(new LibraryUnit(unit, parts));
     }
   }
 
-  static const String _propertyName = 'dev_compiler.BatchCompiler.isCompiled';
-
   void compileHtml(Source source) {
     // TODO(jmesserly): reuse DartScriptsTask instead of copy/paste.
     var contents = context.getContents(source);
     var document = html.parse(contents.data, generateSpans: true);
     var scripts = document.querySelectorAll('script[type="application/dart"]');
 
     var loadedLibs = new LinkedHashSet<Uri>();
 
     for (var script in scripts) {
       Source scriptSource = null;
@@ skipping 240 matching lines @@
     '_rtti.js',
     '_classes.js',
     '_operations.js',
     'dart_runtime.js',
   ];
   files.addAll(corelibOrder.map((l) => l.replaceAll('.', '/') + '.js'));
   return files;
 }();
 
 final _log = new Logger('dev_compiler.src.compiler');