Do not use C++ const references in the C interface.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 526208420b9d7e2ffe854be82aa9b9a744cfa8d9..0ba7d3896e705a98760079a3def114b0488ad4c3 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -102,7 +102,7 @@ typedef void (*Dart_PeerFinalizer)(void* peer);
  *
  * Requires there to be a current isolate.
  */
-DART_EXPORT bool Dart_IsError(const Dart_Handle& handle);
+DART_EXPORT bool Dart_IsError(Dart_Handle handle);
 
 /**
  * Gets the error message from an error handle.
@@ -114,7 +114,7 @@ DART_EXPORT bool Dart_IsError(const Dart_Handle& handle);
  *   String is scope allocated and is only valid until the next call
  *   to Dart_ExitScope.
 */
-DART_EXPORT const char* Dart_GetError(const Dart_Handle& handle);
+DART_EXPORT const char* Dart_GetError(Dart_Handle handle);
 
 /**
  * Is this an error handle for an unhandled exception?