Revert of Remove RegExp.prototype.source getter compat workaround

test/mjsunit/es6/regexp-flags.js

Index: test/mjsunit/es6/regexp-flags.js
diff --git a/test/mjsunit/es6/regexp-flags.js b/test/mjsunit/es6/regexp-flags.js
index 33d103618fd1453e9502e627f48a0941b4ffdbb8..79b0197e917630c35481dd9f5256da7db484a932 100644
--- a/test/mjsunit/es6/regexp-flags.js
+++ b/test/mjsunit/es6/regexp-flags.js
@@ -52,11 +52,7 @@
 function testName(name) {
   // TODO(littledan): For web compatibility, we don't throw an exception,
   // but ES2015 expects an exception to be thrown from this getter.
-  if (name === "source") {
-    assertThrows(() => RegExp.prototype[name], TypeError);
-  } else {
-    assertEquals(undefined, RegExp.prototype[name]);
-  }
+  assertEquals(undefined, RegExp.prototype[name]);
   assertEquals(
       "get " + name,
       Object.getOwnPropertyDescriptor(RegExp.prototype, name).get.name);