In FunctionDefinitionInstantiation in non-strict mode we set the LexicalEnvironment of a function to a NewDeclarativeEnvironment() which correctly initializes [[DisposeCapability]] to a NewDisposeCapability(). However, in strict mode we reuse the LexicalEnvironment which happens to be a Function Environment Record that was created by NewFunctionEnvironment(), which does not correctly set its [[DisposeCapability]] slot.

We either need to unconditionally set [[DisposeCapability]] in NewFunctionEnvironment(), or conditionally set it in FunctionDefinitionInstantiation() when we opt to reuse the LexicalEnvironment. I feel the first option is probably cleaner, though.