[es6] Implement spec compliant ToPrimitive in the runtime.

src/date.js

Index: src/date.js
diff --git a/src/date.js b/src/date.js
index 2be8bcb220a7f3ed91ea1fb6943fa79398d5e4a7..44d2f4df1639044f19958d914c858bdc87760508 100644
--- a/src/date.js
+++ b/src/date.js
@@ -364,6 +364,21 @@ function DateToLocaleTimeString() {
 }
 
 
+// 20.3.4.45 Date.prototype [ @@toPrimitive ] ( hint )
+function DateToPrimitive(hint) {
+  if (!IS_SPEC_OBJECT(this)) {
+    throw MakeTypeError(kIncompatibleMethodReceiver,
+                        "Date.prototype [ @@toPrimitive ]", this);
+  }
+  if (hint === "default") {
+    hint = "string";
+  } else if (hint !== "number" && hint !== "string") {
+    throw MakeTypeError(kInvalidHint, hint);
+  }
+  return %OrdinaryToPrimitive(this, hint);
+}
+
+
 // ECMA 262 - 15.9.5.8
 function DateValueOf() {
   CHECK_DATE(this);
@@ -777,9 +792,10 @@ function DateToISOString() {
 }
 
 
+// 20.3.4.37 Date.prototype.toJSON ( key )
 function DateToJSON(key) {
   var o = TO_OBJECT(this);
-  var tv = $defaultNumber(o);
+  var tv = TO_PRIMITIVE_NUMBER(o);
   if (IS_NUMBER(tv) && !NUMBER_IS_FINITE(tv)) {
     return null;
   }
@@ -831,6 +847,9 @@ utils.InstallFunctions(GlobalDate, DONT_ENUM, [
 
 // Set up non-enumerable constructor property of the Date prototype object.
 %AddNamedProperty(GlobalDate.prototype, "constructor", GlobalDate, DONT_ENUM);
+utils.SetFunctionName(DateToPrimitive, symbolToPrimitive);
+%AddNamedProperty(GlobalDate.prototype, symbolToPrimitive, DateToPrimitive,
+                  DONT_ENUM | READ_ONLY);
 
 // Set up non-enumerable functions of the Date prototype object and
 // set their names.