fix #620, infer the input files from sources

test/codegen_expected/language/application_test.js

-//FAILED TO COMPILE
+dart_library.library('language/application_test', null, /* Imports */[
+  'dart_sdk'
+], function load__application_test(exports, dart_sdk) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const application_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  application_test.main = function() {
+  };
+  dart.fn(application_test.main, VoidTodynamic());
+  // Exports:
+  exports.application_test = application_test;
+});