Codegen support for the argument definition test.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2012, 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.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 145 matching lines @@
   }
 
   static HType mapConstantTypeToSsaType(Constant constant) {
     if (constant.isNull()) return HType.NULL;
     if (constant.isBool()) return HType.BOOLEAN;
     if (constant.isInt()) return HType.INTEGER;
     if (constant.isDouble()) return HType.DOUBLE;
     if (constant.isString()) return HType.STRING;
     if (constant.isList()) return HType.READABLE_ARRAY;
     if (constant.isFunction()) return HType.UNKNOWN;
+    // Give a primitive type to the sentinel to avoid Dart semantics
+    // for equality check.
+    if (constant.isSentinel()) return HType.STRING;
     ObjectConstant objectConstant = constant;
     return new HBoundedType.exact(objectConstant.type);
   }
 
   HConstant addConstant(Constant constant) {
     HConstant result = constants[constant];
     if (result === null) {
       HType type = mapConstantTypeToSsaType(constant);
       result = new HConstant.internal(constant, type);
       entry.addAtExit(result);
@@ skipping 2623 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }