Drop support for \Uxxxx unicode literals since this isn't part of the spec.

lex.go

diff --git a/lex.go b/lex.go
index b33e3de..ad66717 100644
--- a/lex.go
+++ b/lex.go
@@ -303,7 +303,7 @@
 		l.appendRune(decodeEscapedCharacter(r))
 		return nil
 
-	case isUnicodeLiteral(r):
+	case atUnicodeLiteral(r):
 		return l.scanUnicodeLiteral()
 
 	case isEOF(r):
@@ -352,6 +352,12 @@
 	}
 }
 
+// atUnicodeLiteral reports whether we are at a unicode literal.
+// The escape character has already been consumed.
+func atUnicodeLiteral(r rune) bool {
+	return r == 'u'
+}
+
 // isComment reports whether we are at the start of a comment.
 func isComment(r rune) bool {
 	return r == '#' || r == '!'
@@ -387,8 +393,4 @@
 // isWhitespace reports whether the rune is a whitespace character.
 func isWhitespace(r rune) bool {
 	return strings.ContainsRune(whitespace, r)
-// isUnicodeLiteral reports whether we are at a unicode literal.
-// The escape character has already been consumed.
-func isUnicodeLiteral(r rune) bool {
-	return r == 'u' || r == 'U'
 }

properties_test.go

diff --git a/properties_test.go b/properties_test.go
index 613e0d2..08cfb1e 100644
--- a/properties_test.go
+++ b/properties_test.go
@@ -62,7 +62,6 @@
 
 func (l *LoadSuite) TestUnicodeLiteralInKey(c *C) {
 	testKeyValue(c, "key\\u2318 = value", "key⌘", "value")
-	testKeyValue(c, "key\\U2318 = value", "key⌘", "value")
 }
 
 func (l *LoadSuite) TestEscapedCharsInValue(c *C) {