Create Namer and Emitter on codegen start.

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

Index: pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
index 07be9d6e3ffdf80188de554a296e86893a3a6c16..bac4f1e2aadc862728da3cfd688d8fd07d9d1d22 100644
--- a/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/startup_emitter/emitter.dart
@@ -14,12 +14,32 @@ import '../../elements/elements.dart'
     show ClassElement, Element, FieldElement, FunctionElement;
 import '../../js/js.dart' as js;
 import '../../js_backend/js_backend.dart' show JavaScriptBackend, Namer;
-import '../js_emitter.dart' show NativeEmitter;
-import '../js_emitter.dart' as emitterTask show Emitter;
+import '../../world.dart' show ClosedWorld;
+import '../js_emitter.dart' show CodeEmitterTask, NativeEmitter;
+import '../js_emitter.dart' as emitterTask show Emitter, EmitterFactory;
 import '../model.dart';
 import '../program_builder/program_builder.dart' show ProgramBuilder;
 import 'model_emitter.dart';
 
+class EmitterFactory implements emitterTask.EmitterFactory {
+  final bool generateSourceMap;
+
+  EmitterFactory({this.generateSourceMap});
+
+  @override
+  String get patchVersion => "startup";
+
+  @override
+  bool get supportsReflection => false;
+
+  @override
+  Emitter createEmitter(
+      CodeEmitterTask task, Namer namer, ClosedWorld closedWorld) {
+    return new Emitter(
+        task.compiler, namer, task.nativeEmitter, generateSourceMap);
+  }
+}
+
 class Emitter implements emitterTask.Emitter {
   final Compiler _compiler;
   final Namer namer;
@@ -37,18 +57,12 @@ class Emitter implements emitterTask.Emitter {
   DiagnosticReporter get reporter => _compiler.reporter;
 
   @override
-  String get patchVersion => "startup";
-
-  @override
   int emitProgram(ProgramBuilder programBuilder) {
     Program program = programBuilder.buildProgram();
     return _emitter.emitProgram(program);
   }
 
   @override
-  bool get supportsReflection => false;
-
-  @override
   bool isConstantInlinedOrAlreadyEmitted(ConstantValue constant) {
     return _emitter.isConstantInlinedOrAlreadyEmitted(constant);
   }