Gracefully handle syntax errors in Parser::PatternRewriter.

src/parsing/pattern-rewriter.cc

Index: src/parsing/pattern-rewriter.cc
diff --git a/src/parsing/pattern-rewriter.cc b/src/parsing/pattern-rewriter.cc
index 6e20282785d0ba3495b2b4b723f69cb6e861bfd6..8ad708988a8794574c160c5aa162d4ede4837968 100644
--- a/src/parsing/pattern-rewriter.cc
+++ b/src/parsing/pattern-rewriter.cc
@@ -50,7 +50,7 @@ void Parser::PatternRewriter::RewriteDestructuringAssignment(
   rewriter.recursion_level_ = 0;
 
   rewriter.RecurseIntoSubpattern(rewriter.pattern_, nullptr);
-  DCHECK(ok);
+  CHECK(ok);
 }
 
 
@@ -575,7 +575,8 @@ void Parser::PatternRewriter::Visit(AstNode* node) { UNREACHABLE(); }
 
 #define NOT_A_PATTERN(Node)                                        \
   void Parser::PatternRewriter::Visit##Node(v8::internal::Node*) { \
-    UNREACHABLE();                                                 \
+    parser_->ReportMessage(MessageTemplate::kUnsupported);         \
+    *ok_ = false;                                                  \
   }
 
 NOT_A_PATTERN(BinaryOperation)