fix #620, infer the input files from sources

test/codegen_expected/language/top_level_multiple_files_test.js

-//FAILED TO COMPILE
+dart_library.library('language/top_level_multiple_files_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__top_level_multiple_files_test(exports, dart_sdk, expect) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const expect$ = expect.expect;
+  const top_level_multiple_files_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  top_level_multiple_files_test.main = function() {
+    expect$.Expect.equals(top_level_multiple_files_test.topLevelVar, 42);
+    expect$.Expect.equals(top_level_multiple_files_test.topLevelMethod(), 87);
+  };
+  dart.fn(top_level_multiple_files_test.main, VoidTodynamic());
+  top_level_multiple_files_test.topLevelVar = 42;
+  top_level_multiple_files_test.topLevelMethod = function() {
+    return 87;
+  };
+  dart.fn(top_level_multiple_files_test.topLevelMethod, VoidTodynamic());
+  // Exports:
+  exports.top_level_multiple_files_test = top_level_multiple_files_test;
+});