Update the Babel plugin to remove empty class constructors

This only happens when it's safe to do so. The exceptions are:
- when the class extends another subclass: removing the constructor would remove the error about the missing super() call
- when there are default parameters, that could have side effects
- when there are destructured prameters, that could have side effects

external/builder/babel-plugin-pdfjs-preprocessor.mjs

@@ -234,6 +234,24 @@ function babelPluginPDFJSPreprocessor(babel, ctx) {
           }
         },
       },
+      ClassMethod(path) {
+        const {
+          node,
+          parentPath: { parent: classNode },
+        } = path;
+        if (
+          // Remove empty constructors. We only do this for
+          // base classes, as the default constructor of derived
+          // classes is not empty (and an empty constructor
+          // must throw at runtime when constructed)
+          node.kind === "constructor" &&
+          node.body.body.length === 0 &&
+          node.params.every(p => p.type === "Identifier") &&
+          !classNode.superClass
+        ) {
+          path.remove();
+        }
+      },
     },
   };
 }

external/builder/fixtures_babel/constructors-expected.js

@@ -0,0 +1,17 @@
+class A {
+  constructor() {
+    console.log("Hi!");
+  }
+}
+class B {
+  constructor(x = console.log("Hi!")) {}
+}
+class C {
+  constructor({
+    x
+  }) {}
+}
+class D {}
+class E extends A {
+  constructor() {}
+}

external/builder/fixtures_babel/constructors.js

@@ -0,0 +1,21 @@
+class A {
+  constructor() {
+    console.log("Hi!");
+  }
+}
+
+class B {
+  constructor(x = console.log("Hi!")) {}
+}
+
+class C {
+  constructor({ x }) {}
+}
+
+class D {
+  constructor(x, y, z) {}
+}
+
+class E extends A {
+  constructor() {}
+}