Incremental compilation of new classes with fields.

dart/pkg/dart2js_incremental/lib/caching_compiler.dart

Index: dart/pkg/dart2js_incremental/lib/caching_compiler.dart
diff --git a/dart/pkg/dart2js_incremental/lib/caching_compiler.dart b/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
index e239cb08e1d216940064d1546ab061cee0698639..c6b3a4a1dabeba1e9c668871fabe3fc3548c2867 100644
--- a/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
+++ b/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
@@ -56,15 +56,19 @@ Future<Compiler> reuseCompiler(
       }
     }
     oldTag.makeCurrent();
-    return new Future.value(
-        new Compiler(
-            inputProvider,
-            outputProvider,
-            diagnosticHandler,
-            libraryRoot,
-            packageRoot,
-            options,
-            environment));
+    compiler = new Compiler(
+        inputProvider,
+        outputProvider,
+        diagnosticHandler,
+        libraryRoot,
+        packageRoot,
+        options,
+        environment);
+    JavaScriptBackend backend = compiler.backend;
+    // Much like a scout, an incremental compiler is always prepared. For
+    // mixins, at least.
+    backend.emitter.oldEmitter.needsMixinSupport = true;
+    return new Future.value(compiler);
   } else {
     for (final task in compiler.tasks) {
       if (task.watch != null) {