Use a shared function to represent type substitutions with dynamic.

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

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index aa75a3e3405929dbd8143756ae73f063860e173d..80fd6f189b397d7fcd2b4218b8eb15150b98605c 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -922,6 +922,20 @@ class OldEmitter implements Emitter {
     output.add(N);
   }
 
+  void emitFunctionThatReturnsNull(CodeOutput output) {
+    output.addBuffer(
+        jsAst.prettyPrint(
+            js.statement(r'''#.# = function() { return null; }''',
+                         [backend.namer.isolateName,
+                          backend.rti.getFunctionThatReturnsNullName]),
+            compiler, monitor: compiler.dumpInfoTask));
+  }
+
+  jsAst.Expression generateFunctionThatReturnsNull() {
+    return js("#.#", [backend.namer.isolateName,
+                      backend.rti.getFunctionThatReturnsNullName]);
+  }
+
   /// Returns the code equivalent to:
   ///   `function(args) { $.startRootIsolate(X.main$closure(), args); }`
   jsAst.Expression buildIsolateSetupClosure(Element appMain,
@@ -1459,6 +1473,8 @@ class OldEmitter implements Emitter {
     mainOutput.add('init()$N$n');
     mainOutput.add('$isolateProperties$_=$_$isolatePropertiesName$N');
 
+    emitFunctionThatReturnsNull(mainOutput);
+
     emitStaticFunctions(task.outputStaticLists[mainOutputUnit]);
 
     List<ClassElement> classes = task.outputClassLists[mainOutputUnit];