dart2js: Use frequency of occurence to sort metadata indices.

pkg/compiler/lib/src/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 1019 matching lines @@
 
 class SsaDeadCodeEliminator extends HGraphVisitor implements OptimizationPhase {
   final String name = "SsaDeadCodeEliminator";
 
   final Compiler compiler;
   final SsaOptimizerTask optimizer;
   SsaLiveBlockAnalyzer analyzer;
   Map<HInstruction, bool> trivialDeadStoreReceivers =
       new Maplet<HInstruction, bool>();
   bool eliminatedSideEffects = false;
+
   SsaDeadCodeEliminator(this.compiler, this.optimizer);
 
   HInstruction zapInstructionCache;
   HInstruction get zapInstruction {
     if (zapInstructionCache == null) {
       // A constant with no type does not pollute types at phi nodes.
       ConstantValue constant =
-          new DummyConstantValue(const TypeMask.nonNullEmpty());
+          new SyntheticConstantValue(
+              SyntheticConstantKind.EMPTY_VALUE,
+              const TypeMask.nonNullEmpty());
       zapInstructionCache = analyzer.graph.addConstant(constant, compiler);
     }
     return zapInstructionCache;
   }
 
   /// Returns true of [foreign] will throw an noSuchMethod error if
   /// receiver is `null` before having any other side-effects.
   bool templateThrowsNSMonNull(HForeignCode foreign, HInstruction receiver) {
     if (foreign.inputs.length < 1) return false;
     if (foreign.inputs.first != receiver) return false;
@@ skipping 1237 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }