[IMP] owl: introduce dev mode

This commit adds a static key 'dev' on QWeb (which acts as a central
point of access in owl internal code), and a 'mode' key on the value
exported by owl to let external code manipulate it.

closes #117

src/index.ts

@@ -12,12 +12,26 @@ export { Observer } from "./observer";
 // themselves in QWeb, otherwise these files will not even be loaded.
 import "./qweb_directives";
 import "./qweb_extensions";
-export { QWeb } from "./qweb_core";
+import { QWeb } from "./qweb_core";
+export { QWeb };
 
 export { connect, Store } from "./store";
 import * as _utils from "./utils";
 
-export const __info__ = {}
-
+export const __info__ = {};
 
+Object.defineProperty(__info__, "mode", {
+  get() {
+    return QWeb.dev ? "dev" : "prod";
+  },
+  set(mode: string) {
+    QWeb.dev = mode === "dev";
+    if (QWeb.dev) {
+      const url = `https://github.com/odoo/owl/blob/master/doc/tooling.md#development-mode`;
+      console.warn(
+        `Owl is running in dev mode.  This is not suitable for production use. See ${url} for more information.`
+      );
+    }
+  }
+});
 export const utils = _utils;

src/qweb_core.ts

@@ -127,6 +127,9 @@ export class QWeb {
   templates: { [name: string]: Template } = {};
   utils = UTILS;
 
+  // dev mode enables better error messages or more costly validations
+  static dev: boolean = false;
+
   // the id field is useful to be able to hash qweb instances.  The current
   // use case is that component's templates are qweb dependant, and need to be
   // able to map a qweb instance to a template name.