Move assert.h/assert.cc from runtime/vm to runtime/platform

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 3187f9040fbe1aae028490499b12ed4f90a26074..0a728f7f98f411996be07ea22f3cc526c83c130c 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -165,7 +165,7 @@ char* File::GetCanonicalPath(const char* pathname) {
     do {
       abs_path = realpath(pathname, NULL);
     } while (abs_path == NULL && errno == EINTR);
-    assert(abs_path == NULL || IsAbsolutePath(abs_path));
+    ASSERT(abs_path == NULL || IsAbsolutePath(abs_path));
   }
   return abs_path;
 }