Don't let HCheck instructions prevent GVN.

sdk/lib/_internal/compiler/implementation/ssa/codegen.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.
 
 part of ssa;
 
 class SsaCodeGeneratorTask extends CompilerTask {
 
   final JavaScriptBackend backend;
 
@@ skipping 602 matching lines @@
     }
   }
 
   void use(HInstruction argument) {
     if (isGenerateAtUseSite(argument)) {
       visitExpression(argument);
     } else if (argument is HCheck && !variableNames.hasName(argument)) {
       HCheck check = argument;
       use(check.checkedInput);
     } else {
+      assert(variableNames.hasName(argument));
       push(new js.VariableUse(variableNames.getName(argument)));
     }
   }
 
   visit(HInstruction node) {
     node.accept(this);
   }
 
   visitExpression(HInstruction node) {
     bool oldIsGeneratingExpression = isGeneratingExpression;
@@ skipping 2379 matching lines @@
   if (leftType.canBeNull() && rightType.canBeNull()) {
     if (left.isConstantNull() || right.isConstantNull() ||
         (leftType.isPrimitive() && leftType == rightType)) {
       return '==';
     }
     return null;
   } else {
     return '===';
   }
 }