Enable including libraries with a preamble via the HTML script tag

Mutating the self variable with Object.create on the window global
makes objects included on js.context within Dart unavailable in the
browser environment. Fix this issue by only preventing encapsulation
via Object.create when necessary and not in the browser.

Additionally, browsers tend to not have CommonJS globals available. To
prevent ReferenceErrors when running in the browser, check if these
globals are available before setting them on self.

lib/preamble.dart

@@ -1,14 +1,14 @@
 library node_preamble;
 
-final _minified = r"""var dartNodePreambleSelf="undefined"!=typeof global?global:window,self=Object.create(dartNodePreambleSelf);if(self.scheduleImmediate="undefined"!=typeof setImmediate?function(e){setImmediate(e)}:function(e){setTimeout(e,0)},self.require=require,self.exports=exports,"undefined"!=typeof process)self.process=process;if("undefined"!=typeof __dirname)self.__dirname=__dirname;if("undefined"!=typeof __filename)self.__filename=__filename;if("undefined"!=typeof Buffer)self.Buffer=Buffer;var dartNodeIsActuallyNode=!dartNodePreambleSelf.window;try{if("undefined"!=typeof WorkerGlobalScope&&dartNodePreambleSelf instanceof WorkerGlobalScope)dartNodeIsActuallyNode=!1;if("undefined"!=typeof process&&process.versions&&process.versions.hasOwnProperty("electron")&&process.versions.hasOwnProperty("node"))dartNodeIsActuallyNode=!0}catch(e){}if(dartNodeIsActuallyNode){var url=("undefined"!=typeof __webpack_require__?__non_webpack_require__:require)("url");Object.defineProperty(self,"location",{value:{get href(){if(url.pathToFileURL)return url.pathToFileURL(process.cwd()).href+"/";else return"file://"+function(){var e=process.cwd();if("win32"!=process.platform)return e;else return"/"+e.replace(/\\/g,"/")}()+"/"}}}),function(){function e(){try{throw new Error}catch(n){var e=n.stack,r=new RegExp("^ *at [^(]*\\((.*):[0-9]*:[0-9]*\\)$","mg"),o=null;do{var t=r.exec(e);if(null!=t)o=t}while(null!=t);return o[1]}}var r=null;Object.defineProperty(self,"document",{value:{get currentScript(){if(null==r)r={src:e()};return r}}})}(),self.dartDeferredLibraryLoader=function(e,r,o){try{load(e),r()}catch(e){o(e)}}}""";
+final _minified = r"""var dartNodePreambleSelf="undefined"!=typeof global?Object.create(global):window,self=dartNodePreambleSelf;if(self.scheduleImmediate="undefined"!=typeof setImmediate?function(e){setImmediate(e)}:function(e){setTimeout(e,0)},"undefined"!=typeof require)self.require=require;if("undefined"!=typeof exports)self.exports=exports;if("undefined"!=typeof process)self.process=process;if("undefined"!=typeof __dirname)self.__dirname=__dirname;if("undefined"!=typeof __filename)self.__filename=__filename;if("undefined"!=typeof Buffer)self.Buffer=Buffer;var dartNodeIsActuallyNode=!dartNodePreambleSelf.window;try{if("undefined"!=typeof WorkerGlobalScope&&dartNodePreambleSelf instanceof WorkerGlobalScope)dartNodeIsActuallyNode=!1;if("undefined"!=typeof process&&process.versions&&process.versions.hasOwnProperty("electron")&&process.versions.hasOwnProperty("node"))dartNodeIsActuallyNode=!0}catch(e){}if(dartNodeIsActuallyNode){var url=("undefined"!=typeof __webpack_require__?__non_webpack_require__:require)("url");Object.defineProperty(self,"location",{value:{get href(){if(url.pathToFileURL)return url.pathToFileURL(process.cwd()).href+"/";else return"file://"+function(){var e=process.cwd();if("win32"!=process.platform)return e;else return"/"+e.replace(/\\/g,"/")}()+"/"}}}),function(){function e(){try{throw new Error}catch(f){var e=f.stack,r=new RegExp("^ *at [^(]*\\((.*):[0-9]*:[0-9]*\\)$","mg"),o=null;do{var t=r.exec(e);if(null!=t)o=t}while(null!=t);return o[1]}}var r=null;Object.defineProperty(self,"document",{value:{get currentScript(){if(null==r)r={src:e()};return r}}})}(),self.dartDeferredLibraryLoader=function(e,r,o){try{load(e),r()}catch(e){o(e)}}}""";
 
 final _normal = r"""
 // make sure to keep this as 'var'
 // we don't want block scoping
 
-var dartNodePreambleSelf = typeof global !== "undefined" ? global : window;
+var dartNodePreambleSelf = typeof global !== "undefined" ? Object.create(global) : window;
 
-var self = Object.create(dartNodePreambleSelf);
+var self = dartNodePreambleSelf;
 
 self.scheduleImmediate = typeof setImmediate !== "undefined"
     ? function (cb) {
@@ -19,8 +19,12 @@ self.scheduleImmediate = typeof setImmediate !== "undefined"
       };
 
 // CommonJS globals.
-self.require = require;
-self.exports = exports;
+if (typeof require !== "undefined") {
+  self.require = require;
+}
+if (typeof exports !== "undefined") {
+  self.exports = exports;
+}
 
 // Node.js specific exports, check to see if they exist & or polyfilled
 

lib/preamble.js

@@ -1,9 +1,9 @@
 // make sure to keep this as 'var'
 // we don't want block scoping
 
-var dartNodePreambleSelf = typeof global !== "undefined" ? global : window;
+var dartNodePreambleSelf = typeof global !== "undefined" ? Object.create(global) : window;
 
-var self = Object.create(dartNodePreambleSelf);
+var self = dartNodePreambleSelf;
 
 self.scheduleImmediate = typeof setImmediate !== "undefined"
     ? function (cb) {
@@ -14,8 +14,12 @@ self.scheduleImmediate = typeof setImmediate !== "undefined"
       };
 
 // CommonJS globals.
-self.require = require;
-self.exports = exports;
+if (typeof require !== "undefined") {
+  self.require = require;
+}
+if (typeof exports !== "undefined") {
+  self.exports = exports;
+}
 
 // Node.js specific exports, check to see if they exist & or polyfilled