readd previous checks - refactor System polyfill

use webpacks "own" System polyfill, do not try to access a previously existing one.
Instead make sure everything will be "shadowed" by the webpack System. thx @sokra for the help!

buildin/system.js

@@ -1,3 +1,7 @@
-/* global System */
-// "fix" for users of "System" global
-module.exports = typeof System === "undefined" ? {} : System;
+// Provide a "System" global.
+module.exports = {
+	// Make sure import is only used as "System.import"
+	import: function() {
+		throw new Error("System.import cannot be used indirectly");
+	}
+};

lib/dependencies/SystemPlugin.js

@@ -26,6 +26,8 @@ class SystemPlugin {
 
 				parser.plugin("typeof System.import", ParserHelpers.toConstantDependency(JSON.stringify("function")));
 				parser.plugin("evaluate typeof System.import", ParserHelpers.evaluateToString("function"));
+				parser.plugin("typeof System", ParserHelpers.toConstantDependency(JSON.stringify("object")));
+				parser.plugin("evaluate typeof System", ParserHelpers.evaluateToString("object"));
 
 				setNotSupported("System.set");
 				setNotSupported("System.get");

test/cases/parsing/issue-2942/index.js

@@ -1,5 +1,5 @@
 it("should polyfill System", function() {
-	if (typeof System.register === "function") {
+	if (typeof System === "object" && typeof System.register === "function") {
 		require("fail");
 	}
 	(typeof System).should.be.eql("object");

test/cases/parsing/typeof/index.js

@@ -45,6 +45,7 @@ it("should not parse filtered stuff", function() {
 	if(typeof module === "undefined") module = require("fail");
 	if(typeof module != "object") module = require("fail");
 	if(typeof exports == "undefined") exports = require("fail");
+	if(typeof System !== "object") exports = require("fail");
 	if(typeof System.import !== "function") exports = require("fail");
 	if(typeof require.include !== "function") require.include("fail");
 	if(typeof require.ensure !== "function") require.ensure(["fail"], function(){});