dart2js: add skeleton of startup-emitter.

pkg/compiler/lib/src/js_emitter/code_emitter_task.dart

Index: pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
diff --git a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
index 0b9cdb2e3b5435c9fe1621785803466f36f6a2cc..d4530ad392d289b3c70a039a4416425046ec21f4 100644
--- a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
+++ b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
@@ -5,6 +5,8 @@
 part of dart2js.js_emitter;
 
 const USE_LAZY_EMITTER = const bool.fromEnvironment("dart2js.use.lazy.emitter");
+const USE_STARTUP_EMITTER =
+    const bool.fromEnvironment("dart2js.use.startup.emitter");
 
 /**
  * Generates the code for all used classes in the program. Static fields (even
@@ -38,9 +40,14 @@ class CodeEmitterTask extends CompilerTask {
         this.namer = namer,
         this.typeTestRegistry = new TypeTestRegistry(compiler) {
     nativeEmitter = new NativeEmitter(this);
-    emitter = USE_LAZY_EMITTER
-        ? new lazy_js_emitter.Emitter(compiler, namer, nativeEmitter)
-        : new full_js_emitter.Emitter(compiler, namer, generateSourceMap, this);
+    if (USE_LAZY_EMITTER) {
+      emitter = new lazy_js_emitter.Emitter(compiler, namer, nativeEmitter);
+    } else if (USE_STARTUP_EMITTER) {
+      emitter = new startup_js_emitter.Emitter(compiler, namer, nativeEmitter);
+    } else {
+      emitter =
+          new full_js_emitter.Emitter(compiler, namer, generateSourceMap, this);
+    }
     metadataCollector = new MetadataCollector(compiler, emitter);
   }