Use `Function` as function type syntax.

pkg/compiler/lib/src/js_backend/constant_emitter.dart

Index: pkg/compiler/lib/src/js_backend/constant_emitter.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_emitter.dart b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
index 31bcccb9c0971aedf79d8526944f2af21ce27a68..531a376e111dc0c153b23062d4ae2b14a43a8e5f 100644
--- a/pkg/compiler/lib/src/js_backend/constant_emitter.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_emitter.dart
@@ -33,8 +33,8 @@ class ConstantEmitter implements ConstantValueVisitor<jsAst.Expression, Null> {
 
   final Compiler compiler;
   final Namer namer;
-  final _ConstantReferenceGenerator constantReferenceGenerator;
-  final _ConstantListGenerator makeConstantList;
+  final jsAst.Expression Function(ConstantValue) constantReferenceGenerator;
+  final jsAst.Expression Function(jsAst.Expression array) makeConstantList;
 
   /**
    * The given [constantReferenceGenerator] function must, when invoked with a
@@ -44,7 +44,7 @@ class ConstantEmitter implements ConstantValueVisitor<jsAst.Expression, Null> {
   ConstantEmitter(
       this.compiler,
       this.namer,
-      jsAst.Expression this.constantReferenceGenerator(ConstantValue constant),
+      jsAst.Expression Function(ConstantValue) this.constantReferenceGenerator,
       this.makeConstantList);
 
   DiagnosticReporter get reporter => compiler.reporter;