fix #620, infer the input files from sources

test/codegen_expected/language/multi_pass2_test.js

-//FAILED TO COMPILE
+dart_library.library('language/multi_pass2_test', null, /* Imports */[
+  'dart_sdk',
+  'expect'
+], function load__multi_pass2_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 multi_pass2_test = Object.create(null);
+  let VoidTodynamic = () => (VoidTodynamic = dart.constFn(dart.definiteFunctionType(dart.dynamic, [])))();
+  multi_pass2_test.Base = class Base extends core.Object {
+    new(value) {
+      this.value = value;
+    }
+  };
+  dart.setSignature(multi_pass2_test.Base, {
+    constructors: () => ({new: dart.definiteFunctionType(multi_pass2_test.Base, [dart.dynamic])})
+  });
+  multi_pass2_test.MultiPass2Test = class MultiPass2Test extends core.Object {
+    static testMain() {
+      let a = new multi_pass2_test.B(5);
+      expect$.Expect.equals(5, a.value);
+    }
+  };
+  dart.setSignature(multi_pass2_test.MultiPass2Test, {
+    statics: () => ({testMain: dart.definiteFunctionType(dart.dynamic, [])}),
+    names: ['testMain']
+  });
+  multi_pass2_test.main = function() {
+    multi_pass2_test.MultiPass2Test.testMain();
+  };
+  dart.fn(multi_pass2_test.main, VoidTodynamic());
+  multi_pass2_test.A = class A extends multi_pass2_test.Base {
+    new(v) {
+      super.new(v);
+    }
+  };
+  dart.setSignature(multi_pass2_test.A, {
+    constructors: () => ({new: dart.definiteFunctionType(multi_pass2_test.A, [dart.dynamic])})
+  });
+  multi_pass2_test.B = class B extends multi_pass2_test.A {
+    new(v) {
+      super.new(v);
+    }
+  };
+  dart.setSignature(multi_pass2_test.B, {
+    constructors: () => ({new: dart.definiteFunctionType(multi_pass2_test.B, [dart.dynamic])})
+  });
+  // Exports:
+  exports.multi_pass2_test = multi_pass2_test;
+});