Add convention around asserts and make it clear that grammars are con…

[szuend]

…text-sensitive.

While working on the proposal-scopes branch and the decoding of scope information, I was contemplating how we could keep the algorithms somewhat readable while also accounting for malformed source maps without having to sprinkle "if condition, optionally report an error" everywhere.

As such I suggest the following:

• We keep the grammar simple and context-sensitive
• We liberally use asserts to express invariant in the format
• Decoders may rewrite asserts as "optionally report an error".

As an example, I'd find the former much easier to read then the latter:

<emu-grammar>
  OriginalScopeStart :
    `B` ScopeFlags ScopeLine ScopeColumn ScopeName? ScopeKind?
</emu-grammar>
<emu-alg>
  1. Let _flags_ be the VLQUnsignedValue of |ScopeFlags|.
  1. Assert: _flags_ & 0x2 = 0x2 iff, |ScopeKind| is present.
  1. Return RelativeName(|ScopeKind|, _accumulator_, _names_).
</emu-alg>

versus

<emu-grammar>
  OriginalScopeStart :
    `B` ScopeFlags ScopeLine ScopeColumn ScopeNameOrKind? ScopeKind?
</emu-grammar>
<emu-alg>
  1. Let _flags_ be the VLQUnsignedValue of |ScopeFlags|.
  1. If _flags_ & 0x2 ≠ 0x2, return *null*.
  1. If |ScopeNameOrKind| is not present, then
    1. optionally report an error.
    1. return *null*.
  1. If _flags_ & 0x1 = 0x1, then
    1. If |ScopeKind| is not present, then
      1. optionally report an error.
      1. return *null*.
    1. Return RelativeName(|ScopeKind|, _accumulator_, _names_).
  1. Return RelativeName(|ScopeNameOrKind|, _accumulator_, _names_).
</emu-alg>

IMO we are specifying a wire format and not a programming language. I eluded to this in the scopes meeting that we shouldn't tailor the specification text so an engineer can follow it line-by-line and implement it, but rather to convey how the format works.

Just a proposal though. If folks feel strongly about keeping it as-is and trying to spell out how to handle the context-sensitive aspects of the grammar in the decoding steps, then we can do that as well :)