generator: avoid redundant source map mappings

lib/6to5/generation/source-map.js

@@ -1,6 +1,7 @@
 module.exports = SourceMap;
 
 var sourceMap = require("source-map");
+var t         = require("../types");
 
 function SourceMap(position, opts, code) {
   this.position = position;
@@ -33,14 +34,22 @@ SourceMap.prototype.mark = function (node, type) {
   var map = this.map;
   if (!map) return; // no source map
 
+  if (t.isProgram(node) || t.isFile(node)) return; // illegal mapping nodes
+
   var position = this.position;
 
+  var generated = {
+    line: position.line,
+    column: position.column
+  };
+
+  var original = loc[type];
+
+  if (generated.line === original.line && generated.column === original.column) return; // nothing to map
+
   map.addMapping({
     source: this.opts.sourceFileName,
-    generated: {
-      line: position.line,
-      column: position.column
-    },
-    original: loc[type]
+    generated: generated,
+    original: original
   });
 };