ES6: Adding support for size to Set and Map

src/collection.js

Index: src/collection.js
diff --git a/src/collection.js b/src/collection.js
index d36fe18fa00dcd5fdae1210e5f9470c53cbc41f3..d5a8fe21a456c3d99bd6572853a9dddc82a04fcd 100644
--- a/src/collection.js
+++ b/src/collection.js
@@ -88,6 +88,15 @@ function SetDelete(key) {
 }
 
 
+function SetGetSize() {
+  if (!IS_SET(this)) {
+    throw MakeTypeError('incompatible_method_receiver',
+                        ['Set.prototype.size', this]);
+  }
+  return %SetGetSize(this);
+}
+
+
 function MapConstructor() {
   if (%_IsConstructCall()) {
     %MapInitialize(this);
@@ -145,6 +154,15 @@ function MapDelete(key) {
 }
 
 
+function MapGetSize() {
+  if (!IS_MAP(this)) {
+    throw MakeTypeError('incompatible_method_receiver',
+                        ['Map.prototype.size', this]);
+  }
+  return %MapGetSize(this);
+}
+
+
 function WeakMapConstructor() {
   if (%_IsConstructCall()) {
     %WeakMapInitialize(this);
@@ -215,6 +233,7 @@ function WeakMapDelete(key) {
   %SetProperty($Map.prototype, "constructor", $Map, DONT_ENUM);
 
   // Set up the non-enumerable functions on the Set prototype object.
+  InstallGetter($Set.prototype, "size", SetGetSize);
   InstallFunctions($Set.prototype, DONT_ENUM, $Array(
     "add", SetAdd,
     "has", SetHas,
@@ -222,6 +241,7 @@ function WeakMapDelete(key) {
   ));
 
   // Set up the non-enumerable functions on the Map prototype object.
+  InstallGetter($Map.prototype, "size", MapGetSize);
   InstallFunctions($Map.prototype, DONT_ENUM, $Array(
     "get", MapGet,
     "set", MapSet,