fix(typings): repair broken type-checking for StringMap

Note that the previous type of StringMap was overly permissive and didn't catch errors.
Also we have to explicitly type empty objects, which is explained here:
https://github.com/Microsoft/TypeScript/issues/5089

Closes #4487

modules/benchpress/test/metric/multi_metric_spec.ts

@@ -77,7 +77,7 @@ class MockMetric extends Metric {
   }
 
   describe(): {[key: string]: string} {
-    var result = {};
+    var result: {[key: string]: string} = {};
     result[this._id] = 'describe';
     return result;
   }