Recognize unmodifiable lists

pkg/compiler/lib/src/ssa/optimize.dart

Index: pkg/compiler/lib/src/ssa/optimize.dart
diff --git a/pkg/compiler/lib/src/ssa/optimize.dart b/pkg/compiler/lib/src/ssa/optimize.dart
index 752019abf06d6f730440ce4dd66bdc66532e82d7..2ae2827869af37741d270a1b5f1e3ad00be2a711 100644
--- a/pkg/compiler/lib/src/ssa/optimize.dart
+++ b/pkg/compiler/lib/src/ssa/optimize.dart
@@ -105,9 +105,12 @@ bool isFixedLength(mask, Compiler compiler) {
   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;