Always enable RTI for incremental compilation.

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 b2b193cdc14503b2de4433445d935d8546b00158..a20e8340dcec0b2d7970071e09ebde20127ad501 100644
--- a/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
+++ b/dart/pkg/dart2js_incremental/lib/caching_compiler.dart
@@ -65,10 +65,20 @@ Future<Compiler> reuseCompiler(
         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);
+
+    Uri core = Uri.parse("dart:core");
+    return compiler.libraryLoader.loadLibrary(core).then((_) {
+      // Likewise, always be prepared for runtimeType support.
+      // TODO(johnniwinther): Add global switch to force RTI.
+      compiler.enabledRuntimeType = true;
+      backend.registerRuntimeType(
+          compiler.enqueuer.resolution, compiler.globalDependencies);
+      return compiler;
+    });
   } else {
     for (final task in compiler.tasks) {
       if (task.watch != null) {