Overhaul tree IR visitor and rename IR classes.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
index f6d3e8840698056fead66175af79706fe407e4a0..a645fb9a7a19fa5cb0166849f8a825c4149eec6e 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes_sexpr.dart
@@ -62,7 +62,7 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
 
   String visitFieldDefinition(FieldDefinition node) {
     String name = node.element.name;
-    if (node.hasInitializer) {
+    if (node.body != null) {
       String body = visit(node.body);
       return '$indentation(FieldDefinition $name () return\n'
              '$body)';
@@ -111,7 +111,7 @@ class SExpressionStringifier extends Indentation implements Visitor<String> {
     return '$indentation(SuperInitializer $target $selector (\n$arguments)';
   }
 
-  String visitRunnableBody(RunnableBody node) {
+  String visitBody(Body node) {
     namer.setReturnContinuation(node.returnContinuation);
     return indentBlock(() => visit(node.body));
   }