Record the generator's strictness for each emitted bytecode
instruction, instead of applying the executable's final strictness to
every instruction during assembly. This lets class heritage and
computed element names run with strict assignment semantics inside
sloppy scripts.
Also create the class self-binding as a strict immutable binding,
matching ClassDefinitionEvaluation, and cover strict class heritage and
computed names in the runtime tests.
ClassDefinitionEvaluation calls MakeConstructor with false for the
writablePrototype argument, so class constructors get a non-writable,
non-enumerable, non-configurable prototype property. We were using the
ordinary function attributes instead, which also allowed static class
elements named "prototype" to redefine the property.
Add LibJS coverage for the constructor and prototype descriptors, and
for static methods/accessors that attempt to define "prototype".
