chore(typings): remove StringMap

This was a poorly typed attempt to mimic TypeScript's index signatures,
which we can use instead.
This eliminates a very strange type that we were exposing to users, but
not re-exporting through our public API.

Fixes #4483

modules/benchpress/src/metric.ts

@@ -1,7 +1,6 @@
 import {bind, Binding} from 'angular2/src/core/di';
 import {Promise, PromiseWrapper} from 'angular2/src/core/facade/async';
 import {BaseException, WrappedException} from 'angular2/src/core/facade/exceptions';
-import {StringMap} from 'angular2/src/core/facade/collection';
 
 /**
  * A metric is measures values
@@ -21,11 +20,11 @@ export abstract class Metric {
    * since the begin call.
    * @param restart: Whether to restart right after this.
    */
-  endMeasure(restart: boolean): Promise<StringMap<string, any>> { throw new BaseException('NYI'); }
+  endMeasure(restart: boolean): Promise<{[key: string]: any}> { throw new BaseException('NYI'); }
 
   /**
    * Describes the metrics provided by this metric implementation.
    * (e.g. units, ...)
    */
-  describe(): StringMap<string, any> { throw new BaseException('NYI'); }
+  describe(): {[key: string]: any} { throw new BaseException('NYI'); }
 }