fixes #259, clone ast before mutation

lib/src/codegen/js_codegen.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.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 2784 matching lines @@
 
   void _addExtensionType(InterfaceType t) {
     if (t.isObject || !_extensionTypes.add(t.element)) return;
     t = fillDynamicTypeArgs(t, rules.provider);
     t.interfaces.forEach(_addExtensionType);
     t.mixins.forEach(_addExtensionType);
     _addExtensionType(t.superclass);
   }
 
   String generateLibrary(LibraryUnit unit) {
+    // Clone the AST first, so we can mutate it.
+    unit = unit.clone();
     var library = unit.library.element.library;
     var fields = findFieldsNeedingStorage(unit);
     var codegen =
         new JSCodegenVisitor(compiler, library, _extensionTypes, fields);
     var module = codegen.emitLibrary(unit);
     var out = compiler.getOutputPath(library.source.uri);
     return writeJsLibrary(module, out, emitSourceMaps: options.emitSourceMaps);
   }
 }
 
@@ skipping 31 matching lines @@
 
 class _JsThisFinder extends JS.BaseVisitor {
   bool found = false;
   visitThis(JS.This node) {
     found = true;
   }
   visitNode(JS.Node node) {
     if (!found) super.visitNode(node);
   }
 }