fix #620, infer the input files from sources

test/codegen_expected/language/library1_test.js

-//FAILED TO COMPILE
+dart_library.library('language/library1_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__library1_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 library1_test = Object.create(null);
+  const library1_lib = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  library1_test.main = function() {
+    library1_test.Library1Test.testMain();
+  };
+  dart.fn(library1_test.main, VoidTodynamic());
+  library1_test.Library1Test = class Library1Test extends core.Object {
+    static testMain() {
+      let a = new library1_lib.A();
+      let s = a.foo();
+      expect$.Expect.equals(s, "foo-rty two");
+    }
+  };
+  dart.setSignature(library1_test.Library1Test, {
+    statics: () => ({testMain: dart.definiteFunctionType(dart.dynamic, [])}),
+    names: ['testMain']
+  });
+  library1_lib.A = class A extends core.Object {
+    new() {
+    }
+    foo() {
+      return "foo-rty two";
+    }
+  };
+  dart.setSignature(library1_lib.A, {
+    constructors: () => ({new: dart.definiteFunctionType(library1_lib.A, [])}),
+    methods: () => ({foo: dart.definiteFunctionType(core.String, [])})
+  });
+  // Exports:
+  exports.library1_test = library1_test;
+  exports.library1_lib = library1_lib;
+});