Replace dead instructions with a constant with no type

sdk/lib/_internal/compiler/implementation/ssa/optimize.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 OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 876 matching lines @@
 class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
   final String name = "SsaDeadCodeEliminator";
 
   final Compiler compiler;
   SsaLiveBlockAnalyzer analyzer;
   bool eliminatedSideEffects = false;
   SsaDeadCodeEliminator(this.compiler);
 
   HInstruction zapInstructionCache;
   HInstruction get zapInstruction {
-    return (zapInstructionCache == null)
-        ? zapInstructionCache = analyzer.graph.addConstantInt(0, compiler)
-        : zapInstructionCache;
+    if (zapInstructionCache == null) {
+      // A constant with no type does not pollute types at phi nodes.
+      Constant constant =
+          new DummyReceiverConstant(const TypeMask.nonNullEmpty());
+      zapInstructionCache = analyzer.graph.addConstant(constant, compiler);
+    }
+    return zapInstructionCache;
   }
 
   /// Returns whether the next throwing instruction that may have side
   /// effects after [instruction], throws [NoSuchMethodError] on the
   /// same receiver of [instruction].
   bool hasFollowingThrowingNSM(HInstruction instruction) {
     HInstruction receiver = instruction.getDartReceiver(compiler);
     HInstruction current = instruction.next;
     do {
       if ((current.getDartReceiver(compiler) == receiver)
@@ skipping 1055 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }