Improve runtime error when a module is not loaded

pkg/dev_compiler/lib/js/legacy/dart_library.js

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /* This file defines the module loader for the dart runtime.
 */
 var dart_library;
 if (!dart_library) {
 dart_library =
   typeof module != "undefined" && module.exports || {};
@@ skipping 49 matching lines @@
       this._imports = imports;
       this._loader = loader;
 
       // Cyclic import detection
       this._state = LibraryLoader.NOT_LOADED;
     }
 
     loadImports() {
       let results = [];
       for (let name of this._imports) {
-        let lib = libraries.get(name);
-        if (!lib) {
-          throwLibraryError('Library not available: ' + name);
-        }
-        results.push(lib.load());
+        results.push(import_(name));
       }
       return results;
     }
 
     load() {
       // Check for cycles
       if (this._state == LibraryLoader.LOADING) {
         throwLibraryError('Circular dependence on library: '
                               + this._name);
       } else if (this._state >= LibraryLoader.READY) {
@@ skipping 60 matching lines @@
     if (result) {
       console.warn('Already loaded ' + name);
       return result;
     }
     result = new LibraryLoader(name, defaultValue, imports, loader);
     libraries.set(name, result);
     return result;
   }
   dart_library.library = library;
 
-  function import_(libraryName) {
-    let loader = libraries.get(libraryName);
-    // TODO(vsm): A user might call this directly from JS (as we do in tests).
-    // We may want a different error type.
-    if (!loader) throwLibraryError('Library not found: ' + libraryName);
-    return loader.load();
+  // Maintain a stack of active imports.  If a requested library/module is not
+  // available, print the stack to show where/how it was requested.
+  let _stack = [];
+  function import_(name) {
+    let lib = libraries.get(name);
+    if (!lib) {
+      let message = 'Module ' + name + ' not loaded in the browser.';
+      if (_stack != []) {
+        message += '\nDependency via:';
+        let indent = '';
+        for (let last = _stack.length - 1; last >= 0; last--) {
+          indent += ' ';
+          message += '\n' + indent + '- ' + _stack[last];
+        }
+      }
+      throwLibraryError(message);
+    }
+    _stack.push(name);
+    let result = lib.load();
+    _stack.pop();
+    return result;
   }
   dart_library.import = import_;
 
   var _currentIsolate = false;
 
   function start(moduleName, libraryName) {
     if (libraryName == null) libraryName = moduleName;
     let library = import_(moduleName)[libraryName];
     let dart_sdk = import_('dart_sdk');
 
     if (!_currentIsolate) {
       // This import is only needed for chrome debugging. We should provide an
       // option to compile without it.
       dart_sdk._debugger.registerDevtoolsFormatter();
 
       // Create isolate.
       _currentIsolate = true;
       dart_sdk._isolate_helper.startRootIsolate(() => {}, []);
     }
 
     library.main();
   }
   dart_library.start = start;
 
 })(dart_library);
 }