Recognize unmodifiable lists

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 81 matching lines @@
             // Run the simplifier to remove unneeded type checks inserted by
             // type propagation.
             new SsaInstructionSimplifier(constantSystem, backend, this, work),
         ];
       }
       phases.forEach(runPhase);
     });
   }
 }
 
+/// Returns `true` if [mask] represents only types that have a length that
+/// cannot change.  The current implementation is conservative for the purpose
+/// of identifying gvn-able lengths and mis-identifies some unions of fixed
+/// length indexables (see TODO) as not fixed length.
 bool isFixedLength(mask, Compiler compiler) {
   ClassWorld classWorld = compiler.world;
   JavaScriptBackend backend = compiler.backend;
   if (mask.isContainer && mask.length != null) {
     // A container on which we have inferred the length.
     return true;
-  } else if (mask.containsOnly(backend.jsFixedArrayClass)
-             || mask.containsOnlyString(classWorld)
-             || backend.isTypedArray(mask)) {
+  }
+  // TODO(sra): Recognize any combination of fixed length indexables.
+  if (mask.containsOnly(backend.jsFixedArrayClass) ||
+      mask.containsOnly(backend.jsUnmodifiableArrayClass) ||
+      mask.containsOnlyString(classWorld) ||
+      backend.isTypedArray(mask)) {
     return true;
   }
   return false;
 }
 
 /**
  * If both inputs to known operations are available execute the operation at
  * compile-time.
  */
 class SsaInstructionSimplifier extends HBaseVisitor
@@ skipping 2158 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }