VM: Const-correctness fixes.

runtime/vm/dart_api_message.cc

Index: runtime/vm/dart_api_message.cc
diff --git a/runtime/vm/dart_api_message.cc b/runtime/vm/dart_api_message.cc
index bec6df921e68ba4f1d24feea123e92eb5ec07c21..182fc7db827ed87b3a0817fc444c7d8ca0d15637 100644
--- a/runtime/vm/dart_api_message.cc
+++ b/runtime/vm/dart_api_message.cc
@@ -232,7 +232,7 @@ Dart_CObject* ApiMessageReader::AllocateDartCObjectVmIsolateObj(intptr_t id) {
       for (intptr_t i = 0; i < vm_isolate_references_.length(); i++) {
         object = vm_isolate_references_.At(i);
         if (object->type == Dart_CObject_kString) {
-          if (strcmp(str, const_cast<char*>(object->value.as_string)) == 0) {
+          if (strcmp(str, object->value.as_string) == 0) {
             return object;
           }
         }