Overhaul tree IR visitor and rename IR classes.

tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart

 // Copyright (c) 2014, 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.
 
 // SExpressionUnstringifier implements the inverse operation to
 // [SExpressionStringifier].
 
 library sexpr_unstringifier;
 
 import 'package:compiler/src/constants/expressions.dart'
@@ skipping 299 matching lines @@
     // continuation
     String contName = tokens.read("return");
     Continuation cont = name2variable[contName];
     assert(cont != null);
 
     // body
     Expression body = parseExpression();
 
     tokens.consumeEnd();
     return new FunctionDefinition(element, thisParameter, parameters,
-        new RunnableBody(body, cont), null, null);
+        new Body(body, cont), null, null);
   }
 
   /// (IsTrue arg)
   Condition parseCondition() {
     // Handles IsTrue only for now.
     tokens.consumeStart(IS_TRUE);
 
     Definition value = name2variable[tokens.read()];
     assert(value != null);
 
@@ skipping 398 matching lines @@
     return new ReifyTypeVar(type);
   }
 
   /// (This)
   This parseThis() {
     tokens.consumeStart(THIS);
     tokens.consumeEnd();
     return new This();
   }
 }