store all undeclared references on Program to better handle let scoping

lib/6to5/traverse/scope.js

@@ -156,10 +156,6 @@ var functionVariableVisitor = {
     // function identifier doesn't belong to this scope
     if (state.blockId && node === state.blockId) return;
 
-    if (t.isIdentifier(node) && t.isReferenced(node, parent) && !scope.has(node.name)) {
-      state.add(node, true);
-    }
-
     // we've ran into a declaration!
     // we'll let the BlockStatement scope deal with `let` declarations unless
     if (t.isDeclaration(node) && !t.isBlockScoped(node)) {
@@ -168,6 +164,14 @@ var functionVariableVisitor = {
   }
 };
 
+var programReferenceVisitor = {
+  enter: function (node, parent, scope, context, add) {
+    if (t.isReferencedIdentifier(node, parent) && !scope.has(node.name)) {
+      add(node, true);
+    }
+  }
+};
+
 var blockVariableVisitor = {
   enter: function (node, parent, scope, context, add) {
     if (t.isBlockScoped(node)) {
@@ -243,6 +247,12 @@ Scope.prototype.getInfo = function () {
     });
   }
 
+  // Program
+
+  if (t.isProgram(block)) {
+    traverse(block, programReferenceVisitor, this, add);
+  }
+
   // Function - params, rest
 
   if (t.isFunction(block)) {