[parser] Remove an overzealous syntax error check

In some [1] cases where a lexical declaration is not allowed, ASI will
cause a `let` at the end of a line to be interpreted as an
identifier. A recent patch [2] to fix up the error messages from
misplaced `let` usage was a little overzealous in triggering
the error, throwing a SyntaxError in this edge case. This patch
restores the ASI behavior, which is permitted in JSC and
SpiderMonkey as well. Thanks to a test262 test from Andre Bargull
for raising this issue.

[1] https://tc39.github.io/ecma262/#sec-expression-statement
[2] https://codereview.chromium.org/2697193007

Bug: v8:3305
Change-Id: I80ae8ad9a8a93389ff1003323f0d3f003e7a8c8e
Reviewed-on: https://chromium-review.googlesource.com/529225Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45817}

src/parsing/parser-base.h

@@ -5122,8 +5122,10 @@ ParserBase<Impl>::ParseExpressionOrLabelledStatement(
       Token::Value next_next = PeekAhead();
       // "let" followed by either "[", "{" or an identifier means a lexical
       // declaration, which should not appear here.
-      if (next_next != Token::LBRACK && next_next != Token::LBRACE &&
-          next_next != Token::IDENTIFIER) {
+      // However, ASI may insert a line break before an identifier or a brace.
+      if (next_next != Token::LBRACK &&
+          ((next_next != Token::LBRACE && next_next != Token::IDENTIFIER) ||
+           scanner_->HasAnyLineTerminatorAfterNext())) {
         break;
       }
       impl()->ReportMessageAt(scanner()->peek_location(),

test/cctest/test-parsing.cc

@@ -1930,25 +1930,26 @@ TEST(NoErrorsLetSloppyAllModes) {
   };
 
   const char* statement_data[] = {
-    "var let;",
-    "var foo, let;",
-    "try { } catch (let) { }",
-    "function let() { }",
-    "(function let() { })",
-    "function foo(let) { }",
-    "function foo(bar, let) { }",
-    "let = 1;",
-    "var foo = let = 1;",
-    "let * 2;",
-    "++let;",
-    "let++;",
-    "let: 34",
-    "function let(let) { let: let(let + let(0)); }",
-    "({ let: 1 })",
-    "({ get let() { 1 } })",
-    "let(100)",
-    NULL
-  };
+      "var let;",
+      "var foo, let;",
+      "try { } catch (let) { }",
+      "function let() { }",
+      "(function let() { })",
+      "function foo(let) { }",
+      "function foo(bar, let) { }",
+      "let = 1;",
+      "var foo = let = 1;",
+      "let * 2;",
+      "++let;",
+      "let++;",
+      "let: 34",
+      "function let(let) { let: let(let + let(0)); }",
+      "({ let: 1 })",
+      "({ get let() { 1 } })",
+      "let(100)",
+      "L: let\nx",
+      "L: let\n{x}",
+      NULL};
 
   RunParserSyncTest(context_data, statement_data, kSuccess);
 }