Use a shared function to represent type substitutions with dynamic.

pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
index b2c52734aebd91b227ec89744a963de413639186..916fcb81d4347c92393337c026135fe8473b3ca4 100644
--- a/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/new_emitter/emitter.dart
@@ -73,6 +73,12 @@ class Emitter implements emitterTask.Emitter {
   }
 
   @override
+  // TODO(herhut): Use a single shared function.
+  js.Expression generateFunctionThatReturnsNull() {
+    return js.js('function() {}');
+  }
+
+  @override
   js.Expression constantReference(ConstantValue value) {
     return _emitter.constantEmitter.reference(value);
   }