Various speed optimizations:
* Further reduced String interning by using more pre-interned
constants in asc.
* Optimized the lookup of literal field node names of type
IdentifierNode by giving them an empty ReferenceValue.
Previously, these resulted in an expense lookup in
ReferenceValue.findUnqualified() which always failed.
* Optimized the implict AS3 namespace by giving it a prefilled
ReferenceValue. Like above, this reduces the load on
ReferenceValue.findUnqualified(), which is a hot spot.
* Removed two unnecessary context lookups for FlowAnalzyer.
* Reduced the load on java.lang.Scanner, but storing the mxml
implicit imports presplit.
* Filled in the ReferenceValue for some of the generated AST's
IdentifierNode's, to further reduce the load on