Fix main handling in dart2js and add tests.

dart/sdk/lib/_internal/compiler/implementation/js_backend/backend.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 js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 2087 matching lines @@
     if (element == invokeOnMethod) {
       compiler.enabledInvokeOn = true;
     }
   }
 
   CodeBuffer codeOf(Element element) {
     return generatedCode.containsKey(element)
         ? jsAst.prettyPrint(generatedCode[element], compiler)
         : null;
   }
+
+  FunctionElement helperForBadMain() => findHelper('badMain');
+
+  FunctionElement helperForMissingMain() => findHelper('missingMain');
+
+  FunctionElement helperForMainArity() {
+    return findHelper('mainHasTooManyParameters');
+  }
 }
 
 class JavaScriptionResolutionCallbacks extends ResolutionCallbacks {
   final JavaScriptBackend backend;
 
   JavaScriptionResolutionCallbacks(this.backend);
 
   void registerBackendStaticInvocation(Element element, Registry registry) {
     registry.registerStaticInvocation(backend.registerBackendUse(element));
   }
@@ skipping 185 matching lines @@
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final Constant constant;
   // TODO(johnniwinther): Change to [Element] when dependency nodes are added.
   final Registry registry;
 
   const Dependency(this.constant, this.registry);
 }