fixes #157, renaming local library identifiers if needed.

test/codegen/expect/BenchmarkBase.js

-var BenchmarkBase;
-(function(exports) {
+var BenchmarkBase, core;
+(function(exports, core) {
   'use strict';
   class Expect extends core.Object {
     static equals(expected, actual) {
       if (!dart.equals(expected, actual)) {
         throw `Values not equal: ${expected} vs ${actual}`;
       }
     }
     static listEquals(expected, actual) {
       if (expected[core.$length] != actual[core.$length]) {
         throw `Lists have different lengths: ${expected[core.$length]} vs ${actual[core.$length]}`;
@@ skipping 46 matching lines @@
       return result;
     }
     report() {
       let score = this.measure();
       core.print(`${this.name}(RunTime): ${score} us.`);
     }
   }
   // Exports:
   exports.Expect = Expect;
   exports.BenchmarkBase = BenchmarkBase;
-})(BenchmarkBase || (BenchmarkBase = {}));
+})(BenchmarkBase || (BenchmarkBase = {}), core);