linter: unn_lambdas: check assignability of constructor types

Basically, in `_visitInstanceCreation`, we make sure that, given an
expression like `(e) => C(e))`, that the function type of the outer
function expression is the same as the function type of `C(e)`.

And little cleanups:

* _extractElementsOfSimpleIdentifiers's returned Iterable is only used
  to check whether it contains something, so change it to a Set.
* In `isFinalNode`, use `unParenthesized` to unwrap parens, rather
  calling recursively.
* In `_Visitor`, we stored a `LinterContext`, but we only need the
  TypeSystem object, so just store that for more terse code later.
* In `_visitInstanceCreation`, remove the local function, `matches`,
  by checking whether all arguments are SimpleIdentifiers.
* Convert `isFinalElement` to an extension getter on `Expression`.
* Remove helper `isTearoffAssignable`. It is more concise and I think
  easier to read to just use TypeSystem.isSubtypeOf on its own.
* When looking at a VariableDeclaration, get the type from the
  declared element, which will include inference, rather than the
  literal type annotation on the declaration.

Fixes https://github.com/dart-lang/linter/issues/4914
Fixes https://github.com/dart-lang/linter/issues/3516

Change-Id: I3e89ee4dc011473511d375b0ad324988232c8c96
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/364628
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Phil Quitslund <pquitslund@google.com>

https://dart.googlesource.com/sdk/+/293469ee22ba8cb62679310cdfb682019ffff5bd