Overhaul tree IR visitor and rename IR classes.

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

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index f7447c328023299b86438afb9289d2e99e43d0a1..2fa219b762295f5f0a533388d3ec2034777f98a4 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -23,7 +23,8 @@ class CodegenBailout {
   }
 }
 
-class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
+class CodeGenerator extends tree_ir.StatementVisitor
+                    with tree_ir.ExpressionVisitor<js.Expression> {
   final CodegenRegistry registry;
 
   final Glue glue;
@@ -595,16 +596,6 @@ class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
     return errorUnsupportedNode(node);
   }
 
-  @override
-  visitFieldInitializer(tree_ir.FieldInitializer node) {
-    return errorUnsupportedNode(node);
-  }
-
-  @override
-  visitSuperInitializer(tree_ir.SuperInitializer node) {
-    return errorUnsupportedNode(node);
-  }
-
   errorUnsupportedNode(tree_ir.DartSpecificNode node) {
     throw "Unsupported node in JS backend: $node";
   }