Added performance.now() polyfill.

src/Three.js

@@ -62,7 +62,31 @@ if ( self.requestAnimationFrame === undefined || self.cancelAnimationFrame === u
 
 		}
 
-	}() );
+	} )();
+
+}
+
+//
+
+if ( self.performance === undefined ) {
+
+	self.performance = {};
+
+}
+
+if ( self.performance.now === undefined ) {
+
+	( function () {
+
+		var start = Date.now();
+
+		self.performance.now = function () {
+
+			return Date.now() - start;
+
+		}
+
+	} )();
 
 }
 

src/core/Clock.js

@@ -18,17 +18,9 @@ THREE.Clock.prototype = {
 
 	constructor: THREE.Clock,
 
-	_now: function () {
-
-		return self.performance !== undefined && self.performance.now !== undefined
-			? self.performance.now()
-			: Date.now();
-
-	},
-
 	start: function () {
 
-		this.startTime = this._now();
+		this.startTime = self.performance.now();
 
 		this.oldTime = this.startTime;
 		this.running = true;
@@ -61,7 +53,7 @@ THREE.Clock.prototype = {
 
 		if ( this.running ) {
 
-			var newTime = this._now();
+			var newTime = self.performance.now();
 
 			diff = 0.001 * ( newTime - this.oldTime );
 			this.oldTime = newTime;