Introduce a BuiltinsConstructStub that sets up new.target and does a [[call]] per ES6 9.3.2

src/js/regexp.js

Index: src/js/regexp.js
diff --git a/src/js/regexp.js b/src/js/regexp.js
index 55466dc03dd3a1e323d263e8473ea0c2023474cd..4a5c7033e92a4b516f65c4f4a546e8842032cc58 100644
--- a/src/js/regexp.js
+++ b/src/js/regexp.js
@@ -16,6 +16,7 @@ var InternalArray = utils.InternalArray;
 var InternalPackedArray = utils.InternalPackedArray;
 var MakeTypeError;
 var splitSymbol = utils.ImportNow("split_symbol");
+var matchSymbol = utils.ImportNow("match_symbol");;
 
 utils.ImportFromExperimental(function(from) {
   FLAG_harmony_tolength = from.FLAG_harmony_tolength;
@@ -43,61 +44,75 @@ var RegExpLastMatchInfo = new InternalPackedArray(
 
 // -------------------------------------------------------------------
 
-// A recursive descent parser for Patterns according to the grammar of
-// ECMA-262 15.10.1, with deviations noted below.
-function DoConstructRegExp(object, pattern, flags) {
-  // RegExp : Called as constructor; see ECMA-262, section 15.10.4.
-  if (IS_REGEXP(pattern)) {
-    if (!IS_UNDEFINED(flags)) throw MakeTypeError(kRegExpFlags);
-    flags = (REGEXP_GLOBAL(pattern) ? 'g' : '')
-        + (REGEXP_IGNORE_CASE(pattern) ? 'i' : '')
-        + (REGEXP_MULTILINE(pattern) ? 'm' : '')
-        + (REGEXP_UNICODE(pattern) ? 'u' : '')
-        + (REGEXP_STICKY(pattern) ? 'y' : '');
-    pattern = REGEXP_SOURCE(pattern);
-  }
+function IsRegExp(o) {
+  if (!IS_OBJECT(o)) return false;
+  var is_regexp = o[matchSymbol];
+  if (!IS_UNDEFINED(is_regexp)) return TO_BOOLEAN(is_regexp);
+  return IS_REGEXP(o);
+}
+
 
+// ES6 section 21.2.3.2.2
+function RegExpInitialize(object, pattern, flags) {
   pattern = IS_UNDEFINED(pattern) ? '' : TO_STRING(pattern);
   flags = IS_UNDEFINED(flags) ? '' : TO_STRING(flags);
-
   %RegExpInitializeAndCompile(object, pattern, flags);
+  return object;
+}
+
+
+function PatternFlags(pattern) {
+  return (REGEXP_GLOBAL(pattern) ? 'g' : '') +
+         (REGEXP_IGNORE_CASE(pattern) ? 'i' : '') +
+         (REGEXP_MULTILINE(pattern) ? 'm' : '') +
+         (REGEXP_UNICODE(pattern) ? 'u' : '') +
+         (REGEXP_STICKY(pattern) ? 'y' : '');
 }
 
 
 function RegExpConstructor(pattern, flags) {
-  if (%_IsConstructCall()) {
-    DoConstructRegExp(this, pattern, flags);
-  } else {
-    // RegExp : Called as function; see ECMA-262, section 15.10.3.1.
-    if (IS_REGEXP(pattern) && IS_UNDEFINED(flags)) {
+  var newtarget = new.target;
+  var pattern_is_regexp = IsRegExp(pattern);
+
+  if (IS_UNDEFINED(newtarget)) {
+    newtarget = GlobalRegExp;
+
+    // ES6 section 21.2.3.1 step 3.b
+    if (pattern_is_regexp && IS_UNDEFINED(flags) &&
+        pattern.constructor === newtarget) {
       return pattern;
     }
-    return new GlobalRegExp(pattern, flags);
   }
+
+  if (IS_REGEXP(pattern)) {
+    if (IS_UNDEFINED(flags)) flags = PatternFlags(pattern);
+    pattern = REGEXP_SOURCE(pattern);
+
+  } else if (pattern_is_regexp) {
+    var input_pattern = pattern;
+    pattern = pattern.source;
+    if (IS_UNDEFINED(flags)) flags = input_pattern.flags;
+  }
+
+  var object = %NewObject(GlobalRegExp, newtarget);
+  return RegExpInitialize(object, pattern, flags);
 }
 
-// Deprecated RegExp.prototype.compile method.  We behave like the constructor
-// were called again.  In SpiderMonkey, this method returns the regexp object.
-// In JSC, it returns undefined.  For compatibility with JSC, we match their
-// behavior.
+
 function RegExpCompileJS(pattern, flags) {
-  // Both JSC and SpiderMonkey treat a missing pattern argument as the
-  // empty subject string, and an actual undefined value passed as the
-  // pattern as the string 'undefined'.  Note that JSC is inconsistent
-  // here, treating undefined values differently in
-  // RegExp.prototype.compile and in the constructor, where they are
-  // the empty string.  For compatibility with JSC, we match their
-  // behavior.
-  if (this == GlobalRegExp.prototype) {
-    // We don't allow recompiling RegExp.prototype.
+  if (!IS_REGEXP(this)) {
     throw MakeTypeError(kIncompatibleMethodReceiver,
-                        'RegExp.prototype.compile', this);
+                        "RegExp.prototype.compile", this);
   }
-  if (IS_UNDEFINED(pattern) && %_ArgumentsLength() != 0) {
-    DoConstructRegExp(this, 'undefined', flags);
-  } else {
-    DoConstructRegExp(this, pattern, flags);
+
+  if (IS_REGEXP(pattern)) {
+    if (!IS_UNDEFINED(flags)) throw MakeTypeError(kRegExpFlags);
+
+    flags = PatternFlags(pattern);
+    pattern = REGEXP_SOURCE(pattern);
   }
+
+  return RegExpInitialize(this, pattern, flags);
 }