Add web compat workarounds for ES2015 RegExp semantics

src/js/regexp.js

Index: src/js/regexp.js
diff --git a/src/js/regexp.js b/src/js/regexp.js
index 086f5e5c5e62b466e8ccf81653a6d4ed76979816..d358ce06b55d6070eef78b9d4f1224630db8fc84 100644
--- a/src/js/regexp.js
+++ b/src/js/regexp.js
@@ -270,8 +270,17 @@ function TrimRegExp(regexp) {
 }
 
 
+const kRegExpPrototypeToString = 12;
+
 function RegExpToString() {
   if (!IS_REGEXP(this)) {
+    // RegExp.prototype.toString() returns '/(?:)/' as a compatibility fix;
+    // a UseCounter is incremented to track it.
+    // TODO(littledan): Remove this workaround or standardize it
+    if (this === GlobalRegExpPrototype) {
+      %IncrementUseCounter(kRegExpPrototypeToString);
+      return '/(?:)/';
+    }
     throw MakeTypeError(kIncompatibleMethodReceiver,
                         'RegExp.prototype.toString', this);
   }
@@ -491,7 +500,8 @@ function RegExpGetSource() {
 // -------------------------------------------------------------------
 
 %FunctionSetInstanceClassName(GlobalRegExp, 'RegExp');
-%FunctionSetPrototype(GlobalRegExp, new GlobalObject());
+const GlobalRegExpPrototype = new GlobalObject();

[Yang, 2015/12/22 06:31:22]

Regexp.js is one of the remaining native scripts that are not in strict mode.
Not sure whether we want legacy const here.

[Dan Ehrenberg, 2015/12/22 07:20:42]

I think this makes sense, since it'll be a clean upgrade to ES2015 const
semantics.

+%FunctionSetPrototype(GlobalRegExp, GlobalRegExpPrototype);
 %AddNamedProperty(
     GlobalRegExp.prototype, 'constructor', GlobalRegExp, DONT_ENUM);
 %SetCode(GlobalRegExp, RegExpConstructor);