Use global System for script type="import"

dist/es6-module-loader-sans-promises.src.js

@@ -1484,7 +1484,10 @@ function logloads(loads) {
         var script = scripts[i];
         if (script.type == 'module') {
           var source = script.innerHTML.substr(1);
-          System.module(source)['catch'](function(err) { setTimeout(function() { throw err; }); });
+          // It is important to reference the global System, rather than the one
+          // in our closure. We want to ensure that downstream users/libraries
+          // can override System w/ custom behavior.
+          __global.System.module(source)['catch'](function(err) { setTimeout(function() { throw err; }); });
         }
       }
     }

dist/es6-module-loader.src.js

@@ -2672,7 +2672,10 @@ function logloads(loads) {
         var script = scripts[i];
         if (script.type == 'module') {
           var source = script.innerHTML.substr(1);
-          System.module(source)['catch'](function(err) { setTimeout(function() { throw err; }); });
+          // It is important to reference the global System, rather than the one
+          // in our closure. We want to ensure that downstream users/libraries
+          // can override System w/ custom behavior.
+          __global.System.module(source)['catch'](function(err) { setTimeout(function() { throw err; }); });
         }
       }
     }

src/system.js

@@ -291,7 +291,10 @@
         var script = scripts[i];
         if (script.type == 'module') {
           var source = script.innerHTML.substr(1);
-          System.module(source)['catch'](function(err) { setTimeout(function() { throw err; }); });
+          // It is important to reference the global System, rather than the one
+          // in our closure. We want to ensure that downstream users/libraries
+          // can override System w/ custom behavior.
+          __global.System.module(source)['catch'](function(err) { setTimeout(function() { throw err; }); });
         }
       }
     }