Move continueWithExpression to IrBuilder.

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/implementation/constants/expressions.dart'
@@ skipping 484 matching lines @@
 
     Primitive recv = name2variable[tokens.read()];
     assert(recv != null);
 
     dart_types.DartType type = new DummyNamedType(tokens.read());
 
     Continuation cont = name2variable[tokens.read()];
     assert(cont != null);
 
     tokens.consumeEnd();
-    return new TypeOperator(operator, recv, type, cont);
+    return new TypeOperator(recv, type, cont, isTypeTest: operator == 'is');
   }
 
   /// (LetPrim name (primitive)) body
   LetPrim parseLetPrim() {
     tokens.consumeStart(LET_PRIM);
 
     // name
     String name = tokens.read();
 
     // (primitive)
@@ skipping 162 matching lines @@
     return new ReifyTypeVar(type);
   }
 
   /// (This)
   This parseThis() {
     tokens.consumeStart(THIS);
     tokens.consumeEnd();
     return new This();
   }
 }