Fix list length optimization bug: do not inline a length access to a constant if the receiver can b…

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 676 matching lines @@
         if (element == compiler.unnamedListConstructor
             && receiver.inputs.length == 1
             && receiver.inputs[0].isInteger()) {
           return receiver.inputs[0];
         }
       } else if (receiver.isConstantList() || receiver.isConstantString()) {
         return graph.addConstantInt(receiver.constant.length, compiler);
       } else {
         var type = receiver.instructionType.computeMask(compiler);
         if (type.isContainer && type.length != null) {
-          return graph.addConstantInt(type.length, compiler);
+          HInstruction constant = graph.addConstantInt(type.length, compiler);
+          if (type.isNullable) {
+            // If the container can be null, we update all uses of the
+            // length access to use the constant instead, but keep the
+            // length access in the graph, to ensure we still have a
+            // null check.
+            node.block.rewrite(node, constant);
+            return node;
+          } else {
+            return constant;
+          }
         }
       }
     }
     return node;
   }
 
   HInstruction visitIndex(HIndex node) {
     if (node.receiver.isConstantList() && node.index.isConstantInteger()) {
       var instruction = node.receiver;
       List<Constant> entries = instruction.constant.entries;
@@ skipping 843 matching lines @@
       // that knows it is not of a specific Type.
     }
 
     for (HIf ifUser in notIfUsers) {
       changeUsesDominatedBy(ifUser.elseBlock, input, convertedType);
       // TODO(ngeoffray): Also change uses for the then block on a HType
       // that knows it is not of a specific Type.
     }
   }
 }