fix #620, infer the input files from sources

test/codegen_expected/varargs.js

 dart_library.library('varargs', null, /* Imports */[
   'dart_sdk'
 ], function load__varargs(exports, dart_sdk) {
   'use strict';
   const core = dart_sdk.core;
   const dart = dart_sdk.dart;
   const dartx = dart_sdk.dartx;
   const varargs = Object.create(null);
+  const src__varargs = Object.create(null);
   let dynamicAnddynamicTodynamic = () => (dynamicAnddynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic, dart.dynamic])))();
+  let dynamicTodynamic = () => (dynamicTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [dart.dynamic])))();
   varargs.varargsTest = function(x, ...others) {
     let args = [1, others];
     dart.dsend(x, 'call', ...args);
   };
   dart.fn(varargs.varargsTest, dynamicAnddynamicTodynamic());
   varargs.varargsTest2 = function(x, ...others) {
     let args = [1, others];
     dart.dsend(x, 'call', ...args);
   };
   dart.fn(varargs.varargsTest2, dynamicAnddynamicTodynamic());
+  src__varargs._Rest = class _Rest extends core.Object {
+    new() {
+    }
+  };
+  dart.setSignature(src__varargs._Rest, {
+    constructors: () => ({new: dart.definiteFunctionType(src__varargs._Rest, [])})
+  });
+  src__varargs.rest = dart.const(new src__varargs._Rest());
+  src__varargs.spread = function(args) {
+    dart.throw(new core.StateError('The spread function cannot be called, ' + 'it should be compiled away.'));
+  };
+  dart.fn(src__varargs.spread, dynamicTodynamic());
   // Exports:
   exports.varargs = varargs;
+  exports.src__varargs = src__varargs;
 });