Description:
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ResourceBundle::__construct() uses ures_open(), which performs validity checks on 
the given locale identifier. That's reasonable, as the fallback functionality only 
works with proper locale IDs (example: you pass "de_DE", but no such bundle exist, 
it will then use the bundle "de" if that exists).

With the fallback flag off (third ctor argument), ures_open is still used. This 
won't allow loading of data in invalid identifiers, such as some of ICU's built-in 
data (e.g. "supplementalData").

Attached is a patch that uses ures_openDirect() if the fallback flag is off. Tests 
are also included.