fixes #193, factory constructors as static methods

lib/runtime/dart_runtime.js

Index: lib/runtime/dart_runtime.js
diff --git a/lib/runtime/dart_runtime.js b/lib/runtime/dart_runtime.js
index 67cb4e23f5ed193a4637d85021027ba7086775b3..5761a8e897ef533aa8424e37297e0b2fbffbfb5e 100644
--- a/lib/runtime/dart_runtime.js
+++ b/lib/runtime/dart_runtime.js
@@ -151,12 +151,12 @@ var dart, _js_helper, _js_primitives;
     if (obj instanceof Array && realRuntimeType(index) == core.int) {
       return obj[index];
     }
-    return checkAndCall(obj.get, obj, [index], '[]');
+    return checkAndCall(obj.get, void 0, obj, [index], '[]');
   }
   dart.dindex = dindex;
 
   function dsetindex(obj, index, value) {
-    return checkAndCall(obj.set, obj, [index, value], '[]=');
+    return checkAndCall(obj.set, void 0, obj, [index, value], '[]=');
   }
   dart.dsetindex = dsetindex;
 
@@ -910,7 +910,7 @@ var dart, _js_helper, _js_primitives;
    */
   // TODO(jmesserly): this could be faster
   function map(values) {
-    let map = new collection.LinkedHashMap();
+    let map = collection.LinkedHashMap.new();

[vsm, 2015/05/29 21:08:58]

Hmm, it's too bad the JS user has to now keep track of whether it's a factory or
not as well.  Not sure there's a clean way around it though.  

[Jennifer Messerly, 2015/05/29 21:32:17]

yeah, seems kind of okay because:

* the only way to represent this pattern in JS is with static methods.
* in JS they look like static methods if you go to definition.
* in Dart, they're clearly documented as "factory" and function more like static
methods anyway.

It's not perfect though, agreed.

Here's another option: disallow returning "null" from a factory method, in
essence assert the factory returns a non-nullable type. This would need an extra
runtime check though. Then we'd only need to do this pattern for primitives, to
avoid boxing.

would you prefer that option?

     if (Array.isArray(values)) {
       for (let i = 0, end = values.length - 1; i < end; i += 2) {
         let key = values[i];