No need to tringify source maps before caching

source-map-support works fine if the map is already an object.

Author: novemberborn

lib/babel.js

@@ -70,7 +70,7 @@ if (inputSourceMap) {
 
 // include test file
 var transpiled = babel.transformFileSync(testPath, options);
-sourceMapCache[testPath] = JSON.stringify(transpiled.map);
+sourceMapCache[testPath] = transpiled.map;
 requireFromString(transpiled.code, testPath, {
 appendPaths: module.paths
 });