fix string concat

Author: Kingwl

src/compiler/checker.ts

@@ -4978,7 +4978,8 @@ namespace ts {
                 return true;
             }
             else if (expr.kind === SyntaxKind.BinaryExpression) {
-                return isStringConcatExpression((<BinaryExpression>expr).left) && isStringConcatExpression((<BinaryExpression>expr).right);
+                const binaryExpression = <BinaryExpression>expr;
+                return binaryExpression.operatorToken.kind === SyntaxKind.PlusToken && isStringConcatExpression(binaryExpression.left) && isStringConcatExpression(binaryExpression.right);
             }
             return false;
         }