fix #620, infer the input files from sources

test/codegen_expected/language/lazy_static6_test.js

-//FAILED TO COMPILE
+dart_library.library('language/lazy_static6_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__lazy_static6_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 lazy_static6_test = Object.create(null);
+  let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.functionType(dart.dynamic, [dart.dynamic])))();
+  let dynamicTodynamic$ = () => (dynamicTodynamic$ = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic])))();
+  let dynamicToFn = () => (dynamicToFn = dart.constFn(dart.definiteFunctionType(dynamicTodynamic(), [dart.dynamic])))();
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  dart.defineLazy(lazy_static6_test, {
+    get x() {
+      return dart.fn(t => dart.fn(u => dart.dsend(t, '+', u), dynamicTodynamic$()), dynamicToFn());
+    }
+  });
+  lazy_static6_test.main = function() {
+    expect$.Expect.equals(499, dart.dcall(dart.dcall(lazy_static6_test.x, 498), 1));
+    expect$.Expect.equals(42, dart.dcall(dart.dcall(lazy_static6_test.x, 39), 3));
+  };
+  dart.fn(lazy_static6_test.main, VoidTodynamic());
+  // Exports:
+  exports.lazy_static6_test = lazy_static6_test;
+});