Move continueWithExpression to IrBuilder.

sdk/lib/_internal/compiler/implementation/cps_ir/constant_propagation.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.
 
 part of dart2js.optimizers;
 
 /**
  * Propagates constants throughout the IR, and replaces branches with fixed
  * jumps as well as side-effect free expressions with known constant results.
  * Should be followed by the [ShrinkingReducer] pass.
@@ skipping 474 matching lines @@
   void visitTypeOperator(TypeOperator node) {
     Continuation cont = node.continuation.definition;
     setReachable(cont);
 
     void setValues(_ConstnessLattice updateValue) {
       setValue(node, updateValue);
       Parameter returnValue = cont.parameters[0];
       setValue(returnValue, updateValue);
     }
 
-    if (node.operator != "is") {
+    if (node.isTypeCast) {
       // TODO(jgruber): Add support for `as` casts.
       setValues(_ConstnessLattice.NonConst);
     }
 
     _ConstnessLattice cell = getValue(node.receiver.definition);
     if (cell.isUnknown) {
       return;  // And come back later.
     } else if (cell.isNonConst) {
       setValues(_ConstnessLattice.NonConst);
     } else if (node.type.kind == types.TypeKind.INTERFACE) {
@@ skipping 151 matching lines @@
       return that;
     }
 
     if (this.constant == that.constant) {
       return this;
     }
 
     return NonConst;
   }
 }