fix #620, infer the input files from sources

test/codegen_expected/language/private2_test.js

-//FAILED TO COMPILE
+dart_library.library('language/private2_test', null, /* Imports */[
+  'dart_sdk'
+], function load__private2_test(exports, dart_sdk) {
+  'use strict';
+  const core = dart_sdk.core;
+  const dart = dart_sdk.dart;
+  const dartx = dart_sdk.dartx;
+  const private2_test = Object.create(null);
+  const private2_lib = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  const _f = Symbol('_f');
+  private2_test.A = class A extends core.Object {
+    new() {
+      this[_f] = 42;
+      this.g = 43;
+    }
+  };
+  dart.setSignature(private2_test.A, {
+    constructors: () => ({new: dart.definiteFunctionType(private2_test.A, [])})
+  });
+  private2_lib.B = class B extends private2_test.A {
+    new() {
+      super.new();
+    }
+  };
+  dart.setSignature(private2_lib.B, {
+    constructors: () => ({new: dart.definiteFunctionType(private2_lib.B, [])})
+  });
+  private2_test.C = class C extends private2_lib.B {
+    new() {
+      super.new();
+    }
+  };
+  dart.setSignature(private2_test.C, {
+    constructors: () => ({new: dart.definiteFunctionType(private2_test.C, [])})
+  });
+  private2_test.main = function() {
+    let a = new private2_test.A();
+    core.print(a.g);
+    core.print(a[_f]);
+    let o = new private2_test.C();
+    core.print(o.g);
+    core.print(o[_f]);
+  };
+  dart.fn(private2_test.main, VoidTodynamic());
+  // Exports:
+  exports.private2_test = private2_test;
+  exports.private2_lib = private2_lib;
+});