Use default source path for BabelCompiler errors.

The inputFile.getPathInPackage() path is not very useful for package
files, since it does not contain the name of the package.

This change makes BabelCompiler behave more like the CoffeeCompiler, which
also omits the explicit sourcePath option to the inputFile.error method.

Fixes #5551.

packages/babel-compiler/babel-compiler.js

@@ -60,7 +60,6 @@ BCp.processFilesForTarget = function (inputFiles) {
         if (e.loc) {
           inputFile.error({
             message: e.message,
-            sourcePath: inputFilePath,
             line: e.loc.line,
             column: e.loc.column,
           });