VM: Always return immutable collections from the mirrors.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 34a530792daf74bbf15f65d4950af01d612b0080..0f3c7054de0e7669eae1777a5b517df6710da3ab 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -330,7 +330,8 @@ RawString* String::IdentifierPrettyNameRetainPrivate(const String& name) {
       }
       start = i + 1;
     } else if (name.CharAt(i) == '@') {
-      ASSERT(at_pos == -1);  // Only one @ is supported.
+      // Setters should have only one @ so we know where to put the =.
+      ASSERT(!is_setter || (at_pos == -1));
       at_pos = i;
     }
   }