fixes #415, correct type for map literals

lib/runtime/dart/html.js

 dart_library.library('dart/html', null, /* Imports */[
   "dart/_runtime",
   'dart/math',
   'dart/core',
   'dart/_js_helper',
   'dart/async',
   'dart/collection',
   'dart/_foreign_helper',
   'dart/isolate'
 ], /* Lazy imports */[
@@ skipping 6082 matching lines @@
       let withCredentials = opts && 'withCredentials' in opts ? opts.withCredentials : null;
       let responseType = opts && 'responseType' in opts ? opts.responseType : null;
       let requestHeaders = opts && 'requestHeaders' in opts ? opts.requestHeaders : null;
       let onProgress = opts && 'onProgress' in opts ? opts.onProgress : null;
       let parts = [];
       data.forEach(dart.fn((key, value) => {
         parts[dartx.add](`${core.Uri.encodeQueryComponent(key)}=` + `${core.Uri.encodeQueryComponent(value)}`);
       }, dart.void, [core.String, core.String]));
       let formData = parts[dartx.join]('&');
       if (requestHeaders == null) {
-        requestHeaders = dart.map();
+        requestHeaders = dart.map({}, core.String, core.String);
       }
       requestHeaders.putIfAbsent('Content-Type', dart.fn(() => 'application/x-www-form-urlencoded; charset=UTF-8', core.String, []));
       return HttpRequest.request(url, {method: 'POST', withCredentials: withCredentials, responseType: responseType, requestHeaders: requestHeaders, sendData: formData, onProgress: onProgress});
     }
     static request(url, opts) {
       let method = opts && 'method' in opts ? opts.method : null;
       let withCredentials = opts && 'withCredentials' in opts ? opts.withCredentials : null;
       let responseType = opts && 'responseType' in opts ? opts.responseType : null;
       let mimeType = opts && 'mimeType' in opts ? opts.mimeType : null;
       let requestHeaders = opts && 'requestHeaders' in opts ? opts.requestHeaders : null;
@@ skipping 56 matching lines @@
     static requestCrossOrigin(url, opts) {
       let method = opts && 'method' in opts ? opts.method : null;
       let sendData = opts && 'sendData' in opts ? opts.sendData : null;
       if (dart.notNull(HttpRequest.supportsCrossOrigin)) {
         return dart.as(HttpRequest.request(url, {method: method, sendData: sendData}).then(dart.fn(xhr => {
           return xhr.responseText;
         }, dart.dynamic, [HttpRequest])), async.Future$(core.String));
       }
     }
     get responseHeaders() {
-      let headers = dart.map();
+      let headers = dart.map({}, core.String, core.String);
       let headersString = this.getAllResponseHeaders();
       if (headersString == null) {
         return headers;
       }
       let headersList = headersString[dartx.split]('\r\n');
       for (let header of headersList) {
         if (dart.notNull(header[dartx.isEmpty])) {
           continue;
         }
         let splitIdx = header[dartx.indexOf](': ');
@@ skipping 6516 matching lines @@
   exports.ElementUpgrader = ElementUpgrader;
   exports.NodeValidator = NodeValidator;
   exports.NodeTreeSanitizer = NodeTreeSanitizer;
   exports.UriPolicy = UriPolicy;
   exports.spawnDomUri = spawnDomUri;
   exports.unwrap_jso = unwrap_jso;
   exports.wrap_jso = wrap_jso;
   exports.createCustomUpgrader = createCustomUpgrader;
   exports.getHtmlCreateFunction = getHtmlCreateFunction;
 });