Use skylark-preferred quote char for string literal

We currently have no need to discern between strings quoted with ' or ". While it could be
nice for something one day (and may have been in the past), it's yagni now. Removing the
distinction simplifies string concatenation.

--
PiperOrigin-RevId: 148273400
MOS_MIGRATED_REVID=148273400
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java
index 2e4ae53..85a8f8c 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/ValidationTest.java
@@ -35,7 +35,7 @@
 
   @Test
   public void testAssignmentNotValidLValue() {
-    checkError("cannot assign to ''a''", "'a' = 1");
+    checkError("cannot assign to '\"a\"'", "'a' = 1");
   }
 
   @Test