This PR was created by deleting the sections from Subtypes to (but not including) Reference plus the appendix Algorithmic Subtyping, and inserting the section Subtypes from the null safety update branch plus all the following sections until (but not including) Reference, plus the new version of the appendix Algorithmic Subtyping.

The point is that this PR introduces a large chunk of the new material related to null safety which is introduced into the language specification, and it does not include all the small changes to sections that are mostly the same as before (those changes will be submitted as other PRs later on). In other words, the text before Subtyping is unchanged (apart from the fixes mentioned below), and references from the new material to any location before Subtyping may thus very well refer to obsolete language.

A couple of other updates were necessary in order to make the result LaTeX'able:

• dart.sty was updated to be the version used with the null safety PR (the new version includes a number of declarations that are used by the material about null safety).
• dartLangSpec.tex was updated to use \Delta rather than \Gamma to denote a type variable environment (that's the common notation; we previously used \Gamma, but that's typically a "term" variable environment, so \Delta would be less confusing for readers who also read research papers involving type systems).
• \RawFunctionTypeNamed was updated to allow for the required modifier; a couple of changes were necessary in sections before Subtyping in order to make dartLangSpec.tex LaTeX'able again, but this only involved adding the new LaTeX macro argument {r}, further updates will take place later on (this update just updates stuff from Subtyping and out).
• The symbol used to indicate that a word or phrase is being defined was changed from \diamond to \triangle, because \triangle is now also used over = in order to indicate that a certain term of the form A = B defines A.
• Compile-time errors are now also indicated in the margin (using an \ominus symbol, similar to the symbolic traffic sign meaning "No entrance"), this symbol is used in the new material, that is, nowhere before section Subtyping; for instance, it occurs in section Type Null.

Unfortunately, the diff algorithm in git (and, probably, almost any diff algorithm) does not quite understand that the operation performed here is "delete two big chunks of text and insert replacements that are mostly new material", but it does seem to get on track after a while.