dart2js: HGetLength may become fixed

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.
 
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart' show Selectors;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 885 matching lines @@
       if (receiver.inputs[0].isInteger(closedWorld)) {
         return receiver.inputs[0];
       }
     } else if (receiver.isConstantList() || receiver.isConstantString()) {
       return graph.addConstantInt(receiver.constant.length, closedWorld);
     } else {
       var type = receiver.instructionType;
       if (type.isContainer && type.length != null) {
         HInstruction constant = graph.addConstantInt(type.length, closedWorld);
         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.
+          // 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;
         }
       }
     }
+
+    if (node.isAssignable &&
+        isFixedLength(receiver.instructionType, closedWorld)) {
+      // The input type has changed to fixed-length so change to an unassignable
+      // HGetLength to allow more GVN optimizations.
+      return new HGetLength(receiver, node.instructionType,
+          isAssignable: false);
+    }
     return node;
   }
 
   HInstruction visitIndex(HIndex node) {
     if (node.receiver.isConstantList() && node.index.isConstantInteger()) {
       var instruction = node.receiver;
       List<ConstantValue> entries = instruction.constant.entries;
       instruction = node.index;
       int index = instruction.constant.primitiveValue;
       if (index >= 0 && index < entries.length) {
@@ skipping 1853 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }