add optional undeclared variable check transformer

lib/6to5/transformation/transform.js

@@ -98,6 +98,7 @@ _.each({
   typeofSymbol:              require("./transformers/optional-typeof-symbol"),
   coreAliasing:              require("./transformers/optional-core-aliasing"),
   undefinedToVoid:           require("./transformers/optional-undefined-to-void"),
+  undeclaredVariableCheck:   require("./transformers/optional-undeclared-variable-check"),
 
   // spec
   specPropertyLiterals:         require("./transformers/spec-property-literals"),

lib/6to5/transformation/transformers/optional-undeclared-variable-check.js

@@ -0,0 +1,9 @@
+var t = require("../../types");
+
+exports.optional = true;
+
+exports.Identifier = function (node, parent, scope, context, file) {
+  if (!scope.has(node.name, true)) {
+    throw file.errorWithNode(node, "Reference to undeclared variable");
+  }
+};