Add package global and graph global lookups to new lookup code

[jcollins-g]

This is a batch of changes centered around implementing the new package-global lookups.

• Corrects a parser problem where we were greedily eating const and other prefixes from the names of variables during lookup
• Adds global lookups and packageGlobal lookups
• Filed disallow referring to members of a prefixed library that are hidden #2683 to address users depending on prefix lookups being excessively forgiving
• Fixes an accounting problem in the checker where we were comparing accessors and their enclosingCombos as different -- this isn't true in practice.
• Do not accidentally pick up parameters of the default constructor.

This advances Flutter compliance a few percent to 77% -- biggest issues now are scoping problems around inherited documentation.

[coveralls]

Coverage increased (+0.1%) to 57.781% when pulling 13ddc73 on jcollins-g:package-globals into 9ef58f2 on dart-lang:master.

[srawlins]

Looks great. Lots. of. tests.