Fixes #66 S15.2.4.4_A15: Checking Object.prototype.valueOf when called as a global function.
Issue was due to 11.2.3 Function Calls 6.b in the spec.
> b. Else, the base of ref is an Environment Record
>    Let thisValue be the result of calling the ImplicitThisValue concrete method of
>    GetBase(ref).

Environment Records in Tailspin are represented as plain objects for object
environment records or Activations for declarative environment records. Tailspin did
not provide a way to record the 'provideThis' flag for 10.2.1.2.6 ImplicitThisValue,
in object environment records.

To fix a 'provideThis' flag is added both to Reference and to scope objects and
carried along with 'base'. This is instead of adding it directly to 'base' which would
complicate the many direct uses of 'base'.