Make incremental compilation opt-in.

dart/site/try/src/caching_compiler.dart

 // Copyright (c) 2013, 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 trydart.caching_compiler;
 
 import 'dart:profiler' show
     UserTag;
 
 import 'package:compiler/compiler.dart' show
@@ skipping 51 matching lines @@
   }
   if (diagnosticHandler == null) {
     throw 'Missing diagnosticHandler';
   }
   if (outputProvider == null) {
     outputProvider = NullSink.outputProvider;
   }
   Compiler compiler = cachedCompiler;
   if (compiler == null ||
       compiler.libraryRoot != libraryRoot ||
+      !compiler.hasIncrementalSupport ||
       compiler.hasCrashed ||
       compiler.compilerWasCancelled ||
       compiler.enqueuer.resolution.hasEnqueuedEverything ||
       compiler.deferredLoadTask.splitProgram) {
-    if (compiler != null) {
+    if (compiler != null && compiler.hasIncrementalSupport) {
       print('***FLUSH***');
       if (compiler.hasCrashed) {
         print('Unable to reuse compiler due to crash.');
       } else if (compiler.compilerWasCancelled) {
         print('Unable to reuse compiler due to cancel.');
       } else if (compiler.enqueuer.resolution.hasEnqueuedEverything) {
         print('Unable to reuse compiler due to dart:mirrors.');
       } else if (compiler.deferredLoadTask.splitProgram) {
         print('Unable to reuse compiler due to deferred loading.');
       } else {
@@ skipping 83 matching lines @@
       if (library.isPlatformLibrary ||
           (packagesAreImmutable && library.isPackageLibrary)) {
         compiler.libraries[uri] = library;
         reuseLibrary(compiler.libraryLoader, library);
       }
     });
   }
   oldTag.makeCurrent();
   return compiler;
 }