Update the name of 'PropertyAssignment' to 'LonghandPropertyAssignment'.

'PropertyAssignment' is now the name of the super interface brand we use for all thigns that can
be members of an object literal.
Merge property assignment parsing code.
Remove the 'FunctionBlock' ast kind.

We don't want different AST forms for identical syntactic constructs ('Block' and 'FunctionBock').
This prevents reuse in incremental scenarios and forces the incremental parser to know when to
change between the two.
Don't rewrite function property assignments in the AST.
Remove baseline-hacks from the typewriter.
Emit FunctionPropertyAssignments in their native form when emitting in ES6 mode.
Print out methods in the types of object literals as if they were methods, not function typed properties.
Fix missing return types.

Fixes #1352
CR feedback.
More CR feedback.
Improve names for functions in the checker.
Improve naming of checker methods.
Merge branch 'master' into propertyAssignments
Merge pull request #1349 from Microsoft/propertyAssignments

Property assignments
Fix so 'jake diff' works with PowerShell.
Code Review
Merge pull request #1366 from jbondc/master

Fix so 'jake diff' works with PowerShell.
Merge pull request #1358 from Arnavion/fix-missing-return-types

Fix missing return types.
Add -symbolForPrivates compiler flag.
Emit one __symbol function declaration per file.