[libSyntax] Fix parsing for KeyPath

utils/gyb_syntax_support/ExprNodes.py

@@ -300,7 +300,9 @@ EXPR_NODES = [
     # a.b
     Node('MemberAccessExpr', kind='Expr',
          children=[
-             Child("Base", kind='Expr'),
+             # The base needs to be optional to parse expressions in key paths
+             # like \.a
+             Child("Base", kind='Expr', is_optional=True),
              Child("Dot", kind='PeriodToken'),
              Child("Name", kind='Token'),
              Child('DeclNameArguments', kind='DeclNameArguments',
@@ -496,9 +498,17 @@ EXPR_NODES = [
     Node('KeyPathExpr', kind='Expr',
          children=[
              Child('Backslash', kind='BackslashToken'),
+             Child('RootExpr', kind='IdentifierExpr', is_optional=True), 
              Child('Expression', kind='Expr'),
          ]),
 
+    # The period in the key path serves as the base on which the 
+    # right-hand-side of the key path is evaluated
+    Node('KeyPathBaseExpr', kind='Expr', 
+         children=[
+             Child('Period', kind='PeriodToken'),
+         ]),
+
     # e.g. "a." or "a"
     Node('ObjcNamePiece', kind='Syntax',
          children=[