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/angular2/src/http/enums.ts

@@ -1,4 +1,4 @@
-import {StringMap, StringMapWrapper} from 'angular2/src/core/facade/collection';
+import {StringMapWrapper} from 'angular2/src/core/facade/collection';
 
 /**
  * Supported http methods.

modules/angular2/src/http/headers.ts

@@ -5,7 +5,6 @@ import {
   Map,
   MapWrapper,
   ListWrapper,
-  StringMap
 } from 'angular2/src/core/facade/collection';
 
 /**
@@ -36,7 +35,7 @@ import {
  */
 export class Headers {
   _headersMap: Map<string, string[]>;
-  constructor(headers?: Headers | StringMap<string, any>) {
+  constructor(headers?: Headers | {[key: string]: any}) {
     if (isBlank(headers)) {
       this._headersMap = new Map<string, string[]>();
       return;
@@ -44,7 +43,7 @@ export class Headers {
 
     if (headers instanceof Headers) {
       this._headersMap = (<Headers>headers)._headersMap;
-    } else if (headers instanceof StringMap) {
+    } else /*if (headers instanceof StringMap)*/ {
       this._headersMap = MapWrapper.createFromStringMap<string[]>(headers);
       MapWrapper.forEach(this._headersMap, (v, k) => {
         if (!isListLikeIterable(v)) {