Block-level function declarations Web Legacy Compatibility bug

[Kevin Gibbons]

In ECMAScript 2015, functions declared at the top level of function bodies
and of scripts are var-like and functions declared in blocks are let-like
(visible only in that block), generally speaking. This breaks some
previously common patterns using block-level function declarations, which
were not a part of ECMAScript 5 but were commonly supported in browsers,
albeit with varying semantics. Thus, the ECMAScript 6 spec has an extension
(annex B.3.3) which is intended to minimize this breakage by allowing
function declarations in blocks to *also* be var-like when possible. This
means that they create an additional binding in the containing function's
scope which is assigned when execution reaches the block function
declaration.

However, B.3.3 looks for functions that permit these additional bindings
among the containing function's VarScopedDeclarations, which essentially
consists of top-level function declarations and any var declarations.
Importantly, it does not include any function declarations inside of
blocks, and so will never be applied.

Unfortunately, the function body's LexicallyScopedDeclarations will also
fail to contain function declarations inside of blocks. No existing static
semantic rule will include the desired function declarations; changing the
spec to accommodate this may require substantial additions beyond the scope
of this email.

Also, I observe that these additional compatibility steps are only
performed for function bodies, not scripts. I'm not sure if this is
intentional, but it's a bit counterintuitive: `(function(){ { function
f(){} } f(); })();` continues to work with no reference errors, but `{
function f(){} } f();` does not.
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