Make sure a constant that was used before but removed from the graph gets inserted back in the grap…

sdk/lib/_internal/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.
 
 part of ssa;
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
@@ skipping 162 matching lines @@
     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);
       constants[constant] = result;
+    } else if (result.block == null) {
+      // The constant was not used anymore.
+      entry.addAtExit(result);
     }
     return result;
   }
 
   HConstant addConstantInt(int i, ConstantSystem constantSystem) {
     return addConstant(constantSystem.createInt(i));
   }
 
   HConstant addConstantDouble(double d, ConstantSystem constantSystem) {
     return addConstant(constantSystem.createDouble(d));
@@ skipping 2749 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);
 }