small refactor to move JsPeer code out of _finishClassDef
small refactor to move JsPeer code out of _finishClassDef

Nothing wrong with it there, but it was tangled up with the _emitClassIfNeeded code I'm trying to fix for use-before-define issues. Also _finishClassDef's reason for existing is shared code between FunctionTypeAlias/ClassTypeAlias/ClassDeclaration, so the fact that only one of those cares about jsPeer was a sign.

(My first thought was actually _finishClassMembers, but discovered the generics issue: https://github.com/dart-lang/dev_compiler/issues/154)

R=jacobr@google.com

Review URL: https://codereview.chromium.org/1061543004

ochafik/dev_compiler,ochafik/dev_compiler,ochafik/dev_compiler,ochafik/dev_compiler,ronlobo/dev_compiler,ronlobo/dev_compiler,ronlobo/dev_compiler,ronlobo/dev_compiler