diff --git a/src/runtime.js b/src/runtime.js
index 4b600df..14ff1b6 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -48,6 +48,7 @@
 const $Function = global.Function;
 const $Boolean = global.Boolean;
 const $NaN = 0/0;
+const builtins = this;
 
 // ECMA-262 Section 11.9.3.
 function EQUALS(y) {
@@ -365,7 +366,7 @@
 // an expensive ToBoolean conversion in the generated code.
 function INSTANCE_OF(F) {
   var V = this;
-  if (!IS_FUNCTION(F)) {
+  if (!IS_SPEC_FUNCTION(F)) {
     throw %MakeTypeError('instanceof_function_expected', [V]);
   }
 
@@ -407,7 +408,7 @@
   if (!IS_FUNCTION(delegate)) {
     throw %MakeTypeError('called_non_callable', [typeof this]);
   }
-  return delegate.apply(this, arguments);
+  return %Apply(delegate, this, arguments, 0, %_ArgumentsLength());
 }
 
 
@@ -416,7 +417,32 @@
   if (!IS_FUNCTION(delegate)) {
     throw %MakeTypeError('called_non_callable', [typeof this]);
   }
-  return delegate.apply(this, arguments);
+  return %Apply(delegate, this, arguments, 0, %_ArgumentsLength());
+}
+
+
+function CALL_FUNCTION_PROXY() {
+  var arity = %_ArgumentsLength() - 1;
+  var proxy = %_Arguments(arity);  // The proxy comes in as an additional arg.
+  var trap = %GetCallTrap(proxy);
+  return %Apply(trap, this, arguments, 0, arity);
+}
+
+
+function CALL_FUNCTION_PROXY_AS_CONSTRUCTOR(proxy) {
+  var arity = %_ArgumentsLength() - 1;
+  var trap = %GetConstructTrap(proxy);
+  var receiver = void 0;
+  if (!IS_UNDEFINED(trap)) {
+    trap = %GetCallTrap(proxy);
+    var proto = proxy.prototype;
+    if (!IS_SPEC_OBJECT(proto) && proto !== null) {
+      throw MakeTypeError("proto_object_or_null", [proto]);
+    }
+    receiver = new global.Object();
+    receiver.__proto__ = proto;
+  }
+  return %Apply(trap, this, arguments, 1, arity);
 }
 
 
@@ -427,7 +453,8 @@
   // that takes care of more eventualities.
   if (IS_ARRAY(args)) {
     length = args.length;
-    if (%_IsSmi(length) && length >= 0 && length < 0x800000 && IS_FUNCTION(this)) {
+    if (%_IsSmi(length) && length >= 0 && length < 0x800000 &&
+        IS_SPEC_FUNCTION(this)) {
       return length;
     }
   }
@@ -441,7 +468,7 @@
     throw %MakeRangeError('stack_overflow', []);
   }
 
-  if (!IS_FUNCTION(this)) {
+  if (!IS_SPEC_FUNCTION(this)) {
     throw %MakeTypeError('apply_non_function', [ %ToString(this), typeof this ]);
   }
 
@@ -609,13 +636,13 @@
 // ECMA-262, section 8.6.2.6, page 28.
 function DefaultNumber(x) {
   var valueOf = x.valueOf;
-  if (IS_FUNCTION(valueOf)) {
+  if (IS_SPEC_FUNCTION(valueOf)) {
     var v = %_CallFunction(x, valueOf);
     if (%IsPrimitive(v)) return v;
   }
 
   var toString = x.toString;
-  if (IS_FUNCTION(toString)) {
+  if (IS_SPEC_FUNCTION(toString)) {
     var s = %_CallFunction(x, toString);
     if (%IsPrimitive(s)) return s;
   }
@@ -627,13 +654,13 @@
 // ECMA-262, section 8.6.2.6, page 28.
 function DefaultString(x) {
   var toString = x.toString;
-  if (IS_FUNCTION(toString)) {
+  if (IS_SPEC_FUNCTION(toString)) {
     var s = %_CallFunction(x, toString);
     if (%IsPrimitive(s)) return s;
   }
 
   var valueOf = x.valueOf;
-  if (IS_FUNCTION(valueOf)) {
+  if (IS_SPEC_FUNCTION(valueOf)) {
     var v = %_CallFunction(x, valueOf);
     if (%IsPrimitive(v)) return v;
   }