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/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);
 }
 
 class SsaOptimizerTask extends CompilerTask {
   final JavaScriptBackend backend;
   SsaOptimizerTask(JavaScriptBackend backend)
     : this.backend = backend,
       super(backend.compiler);
   String get name => 'SSA optimizer';
   Compiler get compiler => backend.compiler;
 
   void runPhases(HGraph graph, List<OptimizationPhase> phases) {
     for (OptimizationPhase phase in phases) {
       runPhase(graph, phase);
     }
   }
 
   void runPhase(HGraph graph, OptimizationPhase phase) {
     phase.visitGraph(graph);
     compiler.tracer.traceGraph(phase.name, graph);
+    assert(graph.isValid());
   }
 
   void optimize(WorkItem work, HGraph graph, bool speculative) {
     ConstantSystem constantSystem = compiler.backend.constantSystem;
     JavaScriptItemCompilationContext context = work.compilationContext;
     HTypeMap types = context.types;
     measure(() {
       List<OptimizationPhase> phases = <OptimizationPhase>[
           // Run trivial constant folding first to optimize
           // some patterns useful for type conversion.
@@ skipping 1438 matching lines @@
     HInstruction receiver = interceptor.receiver;
     for (var user in receiver.usedBy) {
       if (user is HInterceptor && interceptor.dominates(user)) {
         user.interceptedClasses = interceptor.interceptedClasses;
       }
     }
   }
 
   // TODO(ngeoffray): Also implement it for non-intercepted calls.
 }