Fix optimizer bug with external strings.

runtime/vm/redundancy_elimination.cc

Index: runtime/vm/redundancy_elimination.cc
diff --git a/runtime/vm/redundancy_elimination.cc b/runtime/vm/redundancy_elimination.cc
index 4b8b3bf7da7ff10c7cc723de7a50b421db69dd51..93b39517b0ff51043163af464c1fe82616194bb5 100644
--- a/runtime/vm/redundancy_elimination.cc
+++ b/runtime/vm/redundancy_elimination.cc
@@ -550,6 +550,8 @@ class Place : public ValueObject {
       case kImmutableArrayCid:
       case kOneByteStringCid:
       case kTwoByteStringCid:
+      case kExternalOneByteStringCid:
+      case kExternalTwoByteStringCid:
         // Object arrays and strings do not allow accessing them through
         // different types. No need to attach scale.
         return kNoSize;