Allow creating setter-names with symbol literals.

[lrhn]

Dart uses symbols to represent "code identifiers" at run-time. It does so for a number of reasons, one being the ability to not retain the source names at run-time unless necessary.

Symbols are introduced by the system in noSuchMethod invocations, as the member name or named-parameter names. Those can be compared to user supplied symbols to recognize a specific member being (mis-)called.

User symbols are created using either the Symbol constructor or the #... symbol literal.

A symbol literal like #_foo represents a library private name. It is not equal to #_foo that is evaluated in a different library, and it the only thing that is equal to the symbols created by calls of members with that private name.

The Symbol constructor, even when called as const, cannot create library private symbol names. (Some implementations did so anyway, but not all).

However, the only way to create setter names (like foo= for the setter set foo(...)) is to use the Symbol constructor. The #... literal syntax does not allow a trailing =.

That means that there is currently no (not horribly complicated) way to create a private setter-name.

We should allow symbol literals ending in a single =, like #foo=.
(We probably should also allow #foo.bar.baz= since the Symbol constructor accepts those, even though it doesn't correspond to any source name, whereas #foo.bar.baz is a library name).

This is a non-breaking change. The grammar of symbol literals becomes:

<symbolLiteral> ::= `#' (<operator> | (<identifier> (`.' <identifier>)*) `='?)

(The only change is the addition of ​`='?).

This does not conflict with any existing syntax because #foo = 42 is already not valid (the left-hand-side is not an assignable expression). For parsing disambiguation, a symbol literal always extend as far as possible, like it already does to disambiguate #foo.hashCode.

[eernstg]

If we have any intentions at all to go in the direction where the "native syntax" for symbols, that is, the syntax using #, should only create symbols for declared identifiers in the current scope then we might just as well avoid the '.'s.

[leafpetersen]

Previous related discussion on these bugs:

• Decide whether to allow private symbol constants sdk#36433
• No way to create symbol representing private setter name sdk#34904

[eernstg]

The current Dart.g updated with

symbolLiteral
    :    '#' (operator | (identifier ('.' identifier)*) '='?)
    ;

parses all of language_2 and subdirs with no issues. So there is no parsing problem in allowing symbol literals like #_foo=.