[pwrobeldev]

---------- Motivation ----------
In the case of the generated Dart code each function call looked up
for the native function pointer to execute C++ code. The same applied
to property getters and setters.

Instead of caching the obtained pointer to avoid excessive lookups,
the code executed it for each invocation of any function.

Internally, the 'DynamicLibrary.lookupFunction()' executes 'dlsym()' or
similar OS level function that must lock the global linker mutex.

Because Flutter 3.29 or newer executes the Dart code in the main platform
thread, the frequent execution of 'dlsym()' and locking of global mutex
may encounter contention when other threads load dynamic library or look
for symbols.

Frequent lookups cause the following:

• performance penalty is there, because we always need to lookup the symbol
  and lock global mutex
• the locking of mutex can lead to ANRs of the main thread in Flutter 3.29+

---------- Solution ----------
This commit implements caching of the function pointers via usage of private
global variables.

The private variables are initialized lazily in Dart. It means that symbol
is loaded on the first call to the function.

This reduces the possibility of ANRs.