Add constructors to signatures

test/codegen/expect/BenchmarkBase.js

 var BenchmarkBase = dart.defineLibrary(BenchmarkBase, {});
 var core = dart.import(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 ${actual}`;
       }
     }
@@ skipping 55 matching lines @@
       }).bind(this)), 2000);
       this.teardown();
       return result;
     }
     report() {
       let score = this.measure();
       core.print(`${this.name}(RunTime): ${score} us.`);
     }
   }
   dart.setSignature(BenchmarkBase, {
+    constructors: () => ({BenchmarkBase: [BenchmarkBase, [core.String]]}),
     methods: () => ({
       run: [dart.void, []],
       warmup: [dart.void, []],
       exercise: [dart.void, []],
       setup: [dart.void, []],
       teardown: [dart.void, []],
       measure: [core.double, []],
       report: [dart.void, []]
     }),
     statics: () => ({measureFor: [core.double, [core.Function, core.int]]}),
     names: ['measureFor']
   });
   // Exports:
   exports.Expect = Expect;
   exports.BenchmarkBase = BenchmarkBase;
 })(BenchmarkBase, core);