Support messages in assert().

test/splitting/statements.stmt

 40 columns                              |
 >>> single-line assert
 assert("some short string");
 <<<
 assert("some short string");
 >>> wrapped assert
 assert("some very long string that wraps");
 <<<
 assert(
     "some very long string that wraps");
+>>> single-line assert with message
+assert(true, "blah");
+<<<
+assert(true, "blah");
+>>> split assert with message before both
+assert(true, "looong string that wraps");
+<<<
+assert(
+    true, "looong string that wraps");
+>>> split assert with message after first
+assert(veryLongCondition, "long string that wraps");
+<<<
+assert(veryLongCondition,
+    "long string that wraps");
+>>> split assert with message at both
+assert(veryVeryVeryVeryVeryLongCondition, "long string that wraps");
+<<<
+assert(
+    veryVeryVeryVeryVeryLongCondition,
+    "long string that wraps");
 >>> split in do-while condition
 do {} while ("some long string that wraps");
 <<<
 do {} while (
     "some long string that wraps");
 >>> split in switch value
 switch ("a long string that must wrap") {
   case 0:
     return "ok";
 }
 <<<
 switch (
     "a long string that must wrap") {
   case 0:
     return "ok";
 }