dart2js: add skeleton of startup-emitter.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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
  * in classes) are ignored, since they can be treated as non-class elements.
  *
  * The code for the containing (used) methods must exist in the `universe`.
  */
 class CodeEmitterTask extends CompilerTask {
   // TODO(floitsch): the code-emitter task should not need a namer.
   final Namer namer;
@@ skipping 13 matching lines @@
   // tests.
   // The field is set after the program has been emitted.
   /// Contains a list of all classes that are emitted.
   Set<ClassElement> neededClasses;
 
   CodeEmitterTask(Compiler compiler, Namer namer, bool generateSourceMap)
       : super(compiler),
         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);
   }
 
   String get name => 'Code emitter';
 
   /// Returns the closure expression of a static function.
   jsAst.Expression isolateStaticClosureAccess(FunctionElement element) {
     return emitter.isolateStaticClosureAccess(element);
   }
 
@@ skipping 129 matching lines @@
   bool isConstantInlinedOrAlreadyEmitted(ConstantValue constant);
 
   /// Returns the JS code for accessing the given [constant].
   jsAst.Expression constantReference(ConstantValue constant);
 
   /// Returns the JS template for the given [builtin].
   jsAst.Template templateForBuiltin(JsBuiltin builtin);
 
   void invalidateCaches();
 }