fix #620, infer the input files from sources

test/codegen_expected/language/part_test.js

Index: test/codegen_expected/language/part_test.js
diff --git a/test/codegen_expected/language/part_test.js b/test/codegen_expected/language/part_test.js
index 0c6a6854ae66ffc3bb4d224622bd6d0fb857cb23..74611497d643bef7da81ea0728ee648b419cc501 100644
--- a/test/codegen_expected/language/part_test.js
+++ b/test/codegen_expected/language/part_test.js
@@ -1 +1,17 @@
-//FAILED TO COMPILE
+dart_library.library('language/part_test', null, /* Imports */[
+  'dart_sdk'
+], function load__part_test(exports, dart_sdk) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const part_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  part_test.main = function() {
+    core.print(part_test.foo);
+  };
+  dart.fn(part_test.main, VoidTodynamic());
+  part_test.foo = 'foo';
+  // Exports:
+  exports.part_test = part_test;
+});