Now that Google Code Search allows for searches by file name, you can search for MXML and AS3 files. MXML files can be searched for by specifying file:.mxml$. You can search for AS3 files in a similar way, although it will search for AS/AS2 files as well, since the file extension is the same.
Here's an example of a MXML and AS search:
file:.mxml$ tabnavigator
file:.as$ displayobject
There's not a lot of results at the moment, but I expect that Google will improve that in time.