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/MethodLibraryTest.java b/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
index 17599d7..55867c6 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/MethodLibraryTest.java
@@ -363,7 +363,7 @@
                 + LINE_SEPARATOR
                 + "\tFile \"\", line 7, in bar"
                 + LINE_SEPARATOR
-                + "\t\t'test'.index(x)",
+                + "\t\t\"test\".index(x)",
             "def foo():",
             "  bar(1)",
             "def bar(x):",
@@ -421,7 +421,7 @@
                 + LINE_SEPARATOR
                 + "\tFile \"\", line 5, in bar"
                 + LINE_SEPARATOR
-                + "\t\t'test'.index(x)"
+                + "\t\t\"test\".index(x)"
                 + LINE_SEPARATOR
                 + "method string.index(sub: string, start: int, end: int or NoneType) "
                 + "is not applicable "