LibJS: Tweak generated source in 'new Function()' to match ES 2015 spec

ES 5(.1) described parsing of the function body string as:

https://www.ecma-international.org/ecma-262/5.1/#sec-15.3.2.1

7. If P is not parsable as a FormalParameterList[opt] then throw a SyntaxError exception.
8. If body is not parsable as FunctionBody then throw a SyntaxError exception.

We implemented it as building the source string of a complete function
and feeding that to the parser, with the same outcome. ES 2015+ does
exactly that, but with newlines at certain positions:

https://tc39.es/ecma262/#sec-createdynamicfunction

16. Let bodyString be the string-concatenation of 0x000A (LINE FEED), ? ToString(bodyArg), and 0x000A (LINE FEED).
17. Let prefix be the prefix associated with kind in Table 49.
18. Let sourceString be the string-concatenation of prefix, " anonymous(", P, 0x000A (LINE FEED), ") {", bodyString, and "}".

This patch updates the generated source string to match these
requirements. This will make certain edge cases work, e.g.
'new Function("-->")', where the user supplied input must be placed on
its own line to be valid syntax.

Libraries/LibJS/Runtime/FunctionConstructor.cpp

@@ -81,7 +81,7 @@ Value FunctionConstructor::construct(Function&)
         if (vm.exception())
             return {};
     }
-    auto source = String::formatted("function anonymous({}) {{ {} }}", parameters_source, body_source);
+    auto source = String::formatted("function anonymous({}\n) {{\n{}\n}}", parameters_source, body_source);
     auto parser = Parser(Lexer(source));
     auto function_expression = parser.parse_function_node<FunctionExpression>();
     if (parser.has_errors()) {

Libraries/LibJS/Tests/builtins/Function/Function.js

@@ -30,6 +30,8 @@ describe("correct behavior", () => {
         expect(new Function("return typeof Function()")()).toBe("function");
         expect(new Function("x", "return function (y) { return x + y };")(1)(2)).toBe(3);
 
+        expect(new Function("-->")()).toBeUndefined();
+
         expect(new Function().name).toBe("anonymous");
         expect(new Function().toString()).toBe("function anonymous() {\n  ???\n}");
     });
@@ -45,7 +47,7 @@ describe("errors", () => {
             // This is in line with what other engines are reporting.
             .toThrowWithMessage(
                 SyntaxError,
-                "Unexpected token CurlyClose. Expected BracketClose (line: 1, column: 26)"
+                "Unexpected token CurlyClose. Expected BracketClose (line: 4, column: 1)"
             );
     });
 });

Libraries/LibJS/Tests/parser-line-terminators.js

@@ -1,5 +1,11 @@
 /*
 These tests deliberately produce syntax errors to check what line the parser thinks we're on.
+Note that line numbers are higher than you might expect as the parsed code is:
+
+function anonymous(
+) {
+<code>
+}
 
 ⚠ PLEASE MAKE SURE TO NOT LET YOUR EDITOR REMOVE THE LS/PS LINE TERMINATORS!
 */
@@ -7,31 +13,31 @@ These tests deliberately produce syntax errors to check what line the parser thi
 test("LINE FEED is a line terminator", () => {
     expect(() => {
         Function("\n\n@");
-    }).toThrowWithMessage(SyntaxError, "line: 3, column: 1");
+    }).toThrowWithMessage(SyntaxError, "line: 5, column: 1");
 });
 
 test("CARRIAGE RETURN is a line terminator", () => {
     expect(() => {
         Function("\r\r@");
-    }).toThrowWithMessage(SyntaxError, "line: 3, column: 1");
+    }).toThrowWithMessage(SyntaxError, "line: 5, column: 1");
 });
 
 test("LINE SEPARATOR is a line terminator", () => {
     expect(() => {
         Function("

@");
-    }).toThrowWithMessage(SyntaxError, "line: 3, column: 1");
+    }).toThrowWithMessage(SyntaxError, "line: 5, column: 1");
 });
 
 test("PARAGRAPH SEPARATOR is a line terminator", () => {
     expect(() => {
         Function("

@");
-    }).toThrowWithMessage(SyntaxError, "line: 3, column: 1");
+    }).toThrowWithMessage(SyntaxError, "line: 5, column: 1");
 });
 
 test("CR LF is counted as only one line terminator", () => {
     expect(() => {
         Function("\r\n\r\n@");
-    }).toThrowWithMessage(SyntaxError, "line: 3, column: 1");
+    }).toThrowWithMessage(SyntaxError, "line: 5, column: 1");
 });
 
 test("LF/CR are not allowed in string literal", () => {
@@ -49,7 +55,7 @@ test("LS/PS are allowed in string literal", () => {
 test("line terminators can be mixed (but please don't)", () => {
     expect(() => {
         Function("\r
\r\n
\n\r@");
-    }).toThrowWithMessage(SyntaxError, "line: 7, column: 1");
+    }).toThrowWithMessage(SyntaxError, "line: 9, column: 1");
 });
 
 test("all line terminators are valid for line continuations", () => {