fix #620, infer the input files from sources

test/codegen_expected/language/export_double_same_main_test.js

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