Reland "[VM] Add new SymbolConstant to package:kernel/ast.dart"

The canonicalization of symbols needs to take library privacy into
account. Since the Symbol class itself only has a [_name] field but does
not reference in which library the symbol came from, the [_name] must be
mangled.

Mangling is done by backends and so we make a new [SymbolConstant] which
the backends can desugar by doing appropriate mangling and construction
of a [Symbol] instance.

Fixes https://github.com/dart-lang/sdk/issues/34396

Change-Id: I2e13288483c35f875d39eefdb73677b2cc03527a
Reviewed-on: https://dart-review.googlesource.com/74360
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>