fix #620, infer the input files from sources

test/codegen_expected/language/hello_script_test.js

-//FAILED TO COMPILE
+dart_library.library('language/hello_script_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__hello_script_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 hello_script_test = Object.create(null);
+  const hello_script_lib = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  hello_script_test.main = function() {
+    hello_script_lib.HelloLib.doTest();
+    expect$.Expect.equals(18, hello_script_lib.x);
+    core.print("Hello done.");
+  };
+  dart.fn(hello_script_test.main, VoidTodynamic());
+  hello_script_lib.HelloLib = class HelloLib extends core.Object {
+    static doTest() {
+      hello_script_lib.x = 17;
+      expect$.Expect.equals(17, (() => {
+        let x = hello_script_lib.x;
+        hello_script_lib.x = dart.dsend(x, '+', 1);
+        return x;
+      })());
+      core.print("Hello from Lib!");
+    }
+  };
+  dart.setSignature(hello_script_lib.HelloLib, {
+    statics: () => ({doTest: dart.definiteFunctionType(dart.dynamic, [])}),
+    names: ['doTest']
+  });
+  hello_script_lib.x = null;
+  // Exports:
+  exports.hello_script_test = hello_script_test;
+  exports.hello_script_lib = hello_script_lib;
+});