fix corner case in dead_code (#4981)

lib/compress.js

@@ -3638,11 +3638,17 @@ merge(Compressor.prototype, {
     }
 
     function extract_declarations_from_unreachable_code(compressor, stat, target) {
-        if (!(stat instanceof AST_Definitions || stat instanceof AST_LambdaDefinition)) {
+        if (!(stat instanceof AST_DefClass
+            || stat instanceof AST_Definitions
+            || stat instanceof AST_LambdaDefinition)) {
             AST_Node.warn("Dropping unreachable code [{file}:{line},{col}]", stat.start);
         }
         var block;
         stat.walk(new TreeWalker(function(node, descend) {
+            if (node instanceof AST_DefClass) {
+                push(node);
+                return true;
+            }
             if (node instanceof AST_Definitions) {
                 var defns = [];
                 if (node.remove_initializers(compressor, defns)) {

test/compress/classes.js

@@ -260,6 +260,9 @@ block_scoped: {
     expect: {
         "use strict";
         0;
+        {
+            class A {}
+        }
         if (console) {
             class B {}
         }
@@ -269,6 +272,38 @@ block_scoped: {
     node_version: ">=4"
 }
 
+retain_declaration: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        "use strict";
+        var a = "FAIL";
+        try {
+            console.log(function() {
+                return a;
+                class a {}
+            }());
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        "use strict";
+        var a = "FAIL";
+        try {
+            console.log(function() {
+                return a;
+                class a {}
+            }());
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
 drop_extends: {
     options = {
         inline: true,