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 +234,26 @@ function babelPluginPDFJSPreprocessor(babel, ctx) {
           }
         },
       },
+      ClassMethod: {
+        exit(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 +1,23 @@
+class A {
+  constructor() {
+    console.log("Hi!");
+  }
+}
+class B {
+  constructor(x = console.log("Hi!")) {}
+}
+class C {
+  constructor({
+    x
+  }) {}
+}
+class D {}
+class E extends A {
+  constructor() {}
+}
+class F {
+  constructor() {
+    var a = 0;
+  }
+}
+class G {}

external/builder/fixtures_babel/constructors.js

@@ -0,0 +1,37 @@
+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() {}
+}
+
+class F {
+  constructor() {
+    if (PDFJSDev.test('TRUE')) {
+      var a = 0;
+    }
+  }
+}
+
+class G {
+  constructor() {
+    if (PDFJSDev.test('FALSE')) {
+      var a = 0;
+    }
+  }
+}