dart2js: Add --fast-startup flag.

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 b069aaf3a854d08411e19337db4871b241e6c0f5..bea4320d93d488490cd8ec5ab1e8ef1921ac55cc 100644
--- a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
+++ b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
@@ -5,8 +5,6 @@
 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
@@ -35,14 +33,15 @@ class CodeEmitterTask extends CompilerTask {
   /// Contains a list of all classes that are emitted.
   Set<ClassElement> neededClasses;
 
-  CodeEmitterTask(Compiler compiler, Namer namer, bool generateSourceMap)
+  CodeEmitterTask(Compiler compiler, Namer namer, bool generateSourceMap,
+                  bool useStartupEmitter)
       : super(compiler),
         this.namer = namer,
         this.typeTestRegistry = new TypeTestRegistry(compiler) {
     nativeEmitter = new NativeEmitter(this);
     if (USE_LAZY_EMITTER) {
       emitter = new lazy_js_emitter.Emitter(compiler, namer, nativeEmitter);
-    } else if (USE_STARTUP_EMITTER) {
+    } else if (useStartupEmitter) {
       emitter = new startup_js_emitter.Emitter(
           compiler, namer, nativeEmitter, generateSourceMap);
     } else {