Ensure that hash for a symbol is not set to 0 when it is made external.

runtime/platform/assert.h

Index: runtime/platform/assert.h
===================================================================
--- runtime/platform/assert.h	(revision 39255)
+++ runtime/platform/assert.h	(working copy)
@@ -269,7 +269,12 @@
 
 #endif  // if defined(DEBUG)
 
+#define RELEASE_ASSERT(cond)                                                   \
+  do {                                                                         \
+    if (!(cond)) dart::Assert(__FILE__, __LINE__).Fail("expected: %s", #cond); \
+  } while (false)
 
+
 // The COMPILE_ASSERT macro can be used to verify that a compile time
 // expression is true. For example, you could use it to verify the
 // size of a static array: