feat(http): refactor library to work in dart

Mostly internal refactoring needed to make ts2dart and DartAnalyzer happy.

Fixes #2415

modules/angular2/src/facade/lang.dart

@@ -11,6 +11,8 @@ class Math {
   static double random() => _random.nextDouble();
 }
 
+int ENUM_INDEX(value) => value.index;
+
 class CONST {
   const CONST();
 }

modules/angular2/src/facade/lang.ts

@@ -31,6 +31,11 @@ _global.assert = function assert(condition) {
     _global['assert'].call(condition);
   }
 };
+
+export function ENUM_INDEX(value: int): int {
+  return value;
+}
+
 // This function is needed only to properly support Dart's const expressions
 // see https://github.com/angular/ts2dart/pull/151 for more info
 export function CONST_EXPR<T>(expr: T): T {