Fix bug reported by Jon Hylands as well as the test case.

"Change Set:		asMutatorEnh-st
Date:			22 November 2004
Author:			Samuel Tardieu

It is common to use the expression

  (aSymbol, ':') asSymbol

to build the setter from a getter message. For example, Seaside uses it
internally for building callbacks. This changeset adds the #asMutator
message to the Symbol class.

Test included.
"
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