Issues in the section 10.27(Assignable Expressions) of the language specification

[DartBot]

This issue was originally filed by ms...@unipro.ru

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1. Specification says that an invocation of a method is an assignable expression. But first of all, this fact does not follow from the formal grammar.Second of all, this is not true in the current implementation and seems to be a bug in the specification.
2. There is unused definition of 'selector'. Probably it should be moved to the section 10.26 where it is used but not defined.
3. A little laxity: in this section specification states that an expression of the form 'e.id' is equivalent to a getter invocation, but when using 'e.id' in the left hand side of an assignment that would be a setter invocation. The same with super[e]: it's equivalent to super.[]=(e) when using as lhs expression.

[DartBot]

This comment was originally written by drfibonacci@google.com

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Added Area-Language, Triaged labels.

[gbracha]

Set owner to @gbracha.
Added Done label.