fix #620, infer the input files from sources

test/codegen_expected/language/library_juxtaposition_test.js

-//FAILED TO COMPILE
+dart_library.library('language/library_juxtaposition_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__library_juxtaposition_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 library_juxtaposition_test = Object.create(null);
+  const library_juxtaposition_lib = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  library_juxtaposition_test.main = function() {
+    expect$.Expect.equals(library_juxtaposition_lib.c, 47);
+  };
+  dart.fn(library_juxtaposition_test.main, VoidTodynamic());
+  library_juxtaposition_lib.c = 47;
+  library_juxtaposition_test.c = library_juxtaposition_lib.c;
+  // Exports:
+  exports.library_juxtaposition_test = library_juxtaposition_test;
+  exports.library_juxtaposition_lib = library_juxtaposition_lib;
+});