[parser] Correctly handle invalid escapes in adjacent template tokens.

test/cctest/test-parsing.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7143 matching lines @@
     "tag`left${0}\\u{0`",
     "tag`left${0}\\u{0${1}right`",
     "tag`\\u{\\u{0}`",
     "tag`\\u{\\u{0}${0}right`",
     "tag`left${0}\\u{\\u{0}`",
     "tag`left${0}\\u{\\u{0}${1}right`",
     "tag`\\u{110000}`",
     "tag`\\u{110000}${0}right`",
     "tag`left${0}\\u{110000}`",
     "tag`left${0}\\u{110000}${1}right`",
+    "tag` ${tag`\\u`}`",
+    "tag` ``\\u`",
+    "tag`\\u`` `",
+    "tag`\\u``\\u`",
+    "` ${tag`\\u`}`",
+    "` ``\\u`",
     NULL};
   // clang-format on
 
   // No error with flag
   static const ParserFlag flags[] = {kAllowHarmonyTemplateEscapes};
   RunParserSyncTest(context_data, data, kSuccess, NULL, 0, flags,
                     arraysize(flags));
 
   // Still an error without flag
   RunParserSyncTest(context_data, data, kError);
@@ skipping 58 matching lines @@
     "`left${0}\\u{0${1}right`",
     "`\\u{\\u{0}`",
     "`\\u{\\u{0}${0}right`",
     "`left${0}\\u{\\u{0}`",
     "`left${0}\\u{\\u{0}${1}right`",
     "`\\u{110000}`",
     "`\\u{110000}${0}right`",
     "`left${0}\\u{110000}`",
     "`left${0}\\u{110000}${1}right`",
     "`\\1``\\2`",
+    "tag` ${`\\u`}`",
+    "`\\u```",
     NULL};
   // clang-format on
 
   // Error with flag
   static const ParserFlag flags[] = {kAllowHarmonyTemplateEscapes};
   RunParserSyncTest(context_data, data, kError, NULL, 0, flags,
                     arraysize(flags));
 
   // Still an error without flag
   RunParserSyncTest(context_data, data, kError);
@@ skipping 2676 matching lines @@
     // "for await (x of []) async function a() {};",
     // "for await (x of []) async function a() {}; return a;",
     NULL
   };
 
   // clang-format on
   static const ParserFlag always_flags[] = {kAllowHarmonyAsyncIteration};
   RunParserSyncTest(context_data, data, kError, NULL, 0, always_flags,
                     arraysize(always_flags));
 }