Refactor DartTypeVisitor and ElementVisitor.

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

Index: pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
diff --git a/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart b/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
index fb2425407a0f9f7f60648664ab695545405aad92..d156adbda20edc9c3d45a8a033df91bc75e55b42 100644
--- a/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
+++ b/pkg/compiler/lib/src/js_backend/constant_handler_javascript.dart
@@ -213,19 +213,18 @@ class JavaScriptConstantCompiler extends ConstantCompilerBase
 
   void forgetElement(Element element) {
     super.forgetElement(element);
-    element.accept(new ForgetConstantElementVisitor(this));
+    const ForgetConstantElementVisitor().apply(element, this);
     if (element is AstElement && element.hasNode) {
       element.node.accept(new ForgetConstantNodeVisitor(this));
     }
   }
 }
 
-class ForgetConstantElementVisitor extends ElementVisitor {
-  final JavaScriptConstantCompiler constants;
-
-  ForgetConstantElementVisitor(this.constants);
+class ForgetConstantElementVisitor
+    extends BaseElementVisitor<dynamic, JavaScriptConstantCompiler> {
+  const ForgetConstantElementVisitor();
 
-  void visitElement(Element e) {
+  void visitElement(Element e, JavaScriptConstantCompiler constants) {
     for (MetadataAnnotation data in e.metadata) {
       constants.metadataConstantMap.remove(data);
       if (data.hasNode) {
@@ -234,10 +233,11 @@ class ForgetConstantElementVisitor extends ElementVisitor {
     }
   }
 
-  void visitFunctionElement(FunctionElement e) {
-    super.visitFunctionElement(e);
+  void visitFunctionElement(FunctionElement e,
+                            JavaScriptConstantCompiler constants) {
+    super.visitFunctionElement(e, constants);
     if (e.hasFunctionSignature) {
-      e.functionSignature.forEachParameter(this.visit);
+      e.functionSignature.forEachParameter((p) => apply(p, constants));
     }
   }
 }