Polyfills for error .fileName etc writing behavior

- One polyfill to ensure Duktape 1.3.0 behavior

- One polyfill to ensure Duktape 1.4.0 behavior

If the write behavior change is an issue, you can use the first polyfill
to bring back Duktape 1.3.0 behavior.

polyfills/duktape-error-setter-nonwritable.js

@@ -0,0 +1,20 @@
+/*
+ *  Ensure Error .fileName, .lineNumber, and .stack are not directly writable,
+ *  but can be written using Object.defineProperty().  This matches Duktape
+ *  1.3.0 and prior.
+ *
+ *  See: https://github.com/svaarala/duktape/pull/390.
+ */
+
+(function () {
+    var err = new Error('test');
+    err.fileName = 999;
+    if (err.fileName !== 999) { return; }  // already non-writable
+
+    var fn = new Function('');  // nop
+    Object.defineProperties(Error.prototype, {
+        fileName: { set: fn },
+        lineNumber: { set: fn },
+        stack: { set: fn }
+    });
+})();

polyfills/duktape-error-setter-writable.js

@@ -0,0 +1,19 @@
+/*
+ *  Ensure Error .fileName, .lineNumber, and .stack are directly writable
+ *  without having to use Object.defineProperty().  This matches Duktape
+ *  1.4.0 behavior.
+ *
+ *  See: https://github.com/svaarala/duktape/pull/390.
+ */
+
+(function () {
+    var err = new Error('test');
+    err.fileName = 999;
+    if (err.fileName === 999) { return; }  // already writable
+
+    Object.defineProperties(Error.prototype, {
+        fileName: { set: new Function('v', 'Object.defineProperty(this, "fileName", { value: v, writable: true, enumerable: false, configurable: true });') },
+        lineNumber: { set: new Function('v', 'Object.defineProperty(this, "lineNumber", { value: v, writable: true, enumerable: false, configurable: true });') },
+        stack: { set: new Function('v', 'Object.defineProperty(this, "stack", { value: v, writable: true, enumerable: false, configurable: true });') },
+    });
+})();

util/make_dist.sh

@@ -261,6 +261,8 @@ for i in \
 	object-assign.js \
 	performance-now.js \
 	duktape-isfastint.js \
+	duktape-error-setter-writable.js \
+	duktape-error-setter-nonwritable.js \
 	; do
 	cp polyfills/$i $DIST/polyfills/
 done