format flexibility and warnings

lib/compile.js

@@ -140,6 +140,7 @@ function compileOutputs(loader, tree, opts, sfxEntryPoints, cache) {
 }
 
 exports.wrapSFXOutputs = wrapSFXOutputs;
+var formats = ['amd', 'cjs', 'esm', 'global'];
 function wrapSFXOutputs(loader, tree, outputs, sfxEntryPoints, opts) {
   var compilers = {};
 
@@ -148,8 +149,16 @@ function wrapSFXOutputs(loader, tree, outputs, sfxEntryPoints, opts) {
   // determine compilers used
   modules.forEach(function(name) {
     var load = tree[name];
-    if (load.metadata.build !== false)
+    if (load.metadata.build !== false) {
+      var format = load.metadata.format;
+      // NB to be deprecated with SystemJS 0.19
+      if (format == 'es6')
+        format = 'esm';
+      if (!(load.metadata.format in compilerMap))
+        throw new Error(name + ' has format set to "' + load.metadata.format + '", which is not valid. Must be one of "esm", "amd", "cjs", "global" or "register".');
+
       compilers[load.metadata.format] = true;
+    }
   });
 
   // include compiler helpers at the beginning of outputs