Pharo does have something like full text search across all methods, and it works like this: Rightclick the text you'd like to search for, and choose "extended search" from the options. Then, choose "method source with it." A progress bar will appear, and you should get your answer within a minute. That has to do with the source not being loaded into Pharo at all times, it is read on demand out of the sources file.
However, the compiled sources are in memory from the start. While you can't meaningfully search compiled sources, you can search the string literals within. Again, rightclick the text you'd like to search for, and choose "extended search" from the options. But now, choose "method strings with it." That will bring up a list of all methods that contain the selected text as a string literal—instantly.