Compute an incremental patch to JavaScript code.

dart/sdk/lib/_internal/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:async';
 
 import '../compiler.dart' as api;
 import 'dart2jslib.dart' as leg;
 import 'tree/tree.dart' as tree;
 import 'elements/elements.dart' as elements;
 import '../../libraries.dart';
 import 'source_file.dart';
 
+const bool forceIncrementalSupport =
+    const bool.fromEnvironment('DART2JS_EXPERIMENTAL_INCREMENTAL_SUPPORT');
 
 class Compiler extends leg.Compiler {
   api.CompilerInputProvider provider;
   api.DiagnosticHandler handler;
   final Uri libraryRoot;
   final Uri packageRoot;
   List<String> options;
   Map<String, dynamic> environment;
   bool mockableLibraryUsed = false;
   final Set<String> allowedLibraryCategories;
@@ skipping 38 matching lines @@
             sourceMapUri: extractUriOption(options, '--source-map='),
             outputUri: extractUriOption(options, '--out='),
             terseDiagnostics: hasOption(options, '--terse'),
             dumpInfo: hasOption(options, '--dump-info'),
             buildId: extractStringOption(
                 options, '--build-id=',
                 "build number could not be determined"),
             showPackageWarnings:
                 hasOption(options, '--show-package-warnings'),
             useContentSecurityPolicy: hasOption(options, '--csp'),
-            hasIncrementalSupport: hasOption(options, '--incremental-support'),
+            hasIncrementalSupport:
+                forceIncrementalSupport ||
+                hasOption(options, '--incremental-support'),
             suppressWarnings: hasOption(options, '--suppress-warnings')) {
     tasks.addAll([
         userHandlerTask = new leg.GenericTask('Diagnostic handler', this),
         userProviderTask = new leg.GenericTask('Input provider', this),
     ]);
     if (!libraryRoot.path.endsWith("/")) {
       throw new ArgumentError("libraryRoot must end with a /");
     }
     if (packageRoot != null && !packageRoot.path.endsWith("/")) {
       throw new ArgumentError("packageRoot must end with a /");
@@ skipping 259 matching lines @@
   }
 
   void diagnoseCrashInUserCode(String message, exception, stackTrace) {
     hasCrashed = true;
     print('$message: ${tryToString(exception)}');
     print(tryToString(stackTrace));
   }
 
   fromEnvironment(String name) => environment[name];
 }