Removing getSentinel from PTRecordLiteral.
Removing getSentinel from PTMenuLiteral.
Removing getSentinel from PTMenuBarLiteral.
Removing getSentinel from PTMenuItemLiteral.
Removing getSentinel from PTPageLiteral.
Removing getSentinel from PTComponentLiteral.
Removing getSentinel from PTDefnLiteral.
Removing getSentinel from PTCallFrameBoundary, converting it to singleton.
Converted PTString away from use of sentinels and to the use of PTTypeConstraint instead.
Now passing type constraint objects everywhere that sentinels were previously expected.
Creating type constraint for app class objects.
Adding toString to type constraint classes.
Now using type constraints throughout entire system; next step is to create a Null concrete type.
Adding null type; all declared identifiers with object type constraints must be assigned the null singleton.
Cleaning up read-only and removing system write check.
Removing more unnecessary code, including TFlag.
Removing use of Type flag from codebase.
No need to override typeCheck on PTRecordTypeConstraint.
Overriding type check on PTAppClassObjTypeConstraint to ensure prog defn is taken into account.
No need to override typeCheck on PTRowsetTypeConstraint, included comment explaining why.