Compute an incremental patch to JavaScript code.

dart/sdk/lib/_internal/compiler/implementation/apiimpl.dart

Index: dart/sdk/lib/_internal/compiler/implementation/apiimpl.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/apiimpl.dart b/dart/sdk/lib/_internal/compiler/implementation/apiimpl.dart
index 8cb14ddc4d69fafe17373c707ba28f71fe2a1ed0..27672ab2bf90b978d90251a238c2b62f17fd3b5a 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/apiimpl.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/apiimpl.dart
@@ -13,6 +13,8 @@ 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;
@@ -71,7 +73,9 @@ class Compiler extends leg.Compiler {
             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),