Add assertion to check that a break label is identified at compile-ti…

…me (#1852)

This PR changes the following:

- Adds a check at compile time for the existence of a break label (this should be a syntax error in the future; refactor from panics to results in compile should be a separate PR)
- Adds a test for break label existence in boa/tests

262 misses some fairly important JS parity issues and not performing this check eagerly can lead to other more severe issues during VM execution.

boa/src/bytecompiler.rs

@@ -1471,19 +1471,26 @@ impl<'b> ByteCompiler<'b> {
                     self.emit(Opcode::FinallySetJump, &[u32::MAX]);
                 }
                 let label = self.jump();
-                if node.label().is_none() {
-                    self.jump_info
-                        .last_mut()
-                        .expect("no jump information found")
-                        .breaks
-                        .push(label);
-                } else {
+                if let Some(label_name) = node.label() {
+                    let mut found = false;
                     for info in self.jump_info.iter_mut().rev() {
-                        if info.label == node.label() {
+                        if info.label == Some(label_name) {
                             info.breaks.push(label);
+                            found = true;
                             break;
                         }
                     }
+                    assert!(
+                        found,
+                        "Undefined label '{}'",
+                        self.interner().resolve_expect(label_name)
+                    );
+                } else {
+                    self.jump_info
+                        .last_mut()
+                        .expect("no jump information found")
+                        .breaks
+                        .push(label);
                 }
             }
             Node::Block(block) => {

boa/src/tests.rs

@@ -430,6 +430,18 @@ fn for_loop_iteration_variable_does_not_leak() {
     assert_eq!(&exec(inner_scope), "\"i is not defined\"");
 }
 
+#[test]
+#[should_panic]
+fn test_invalid_break_target() {
+    let src = r#"
+        while (false) {
+          break nonexistent;
+        }
+        "#;
+
+    let _ = &exec(src);
+}
+
 #[test]
 fn unary_pre() {
     let unary_inc = r#"