Improve the error message when an import fails.

runtime/bin/builtin.dart

 // Copyright (c) 2012, 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.
 
 library builtin;
 // NOTE: Do not import 'dart:io' in builtin.
 import 'dart:async';
 import 'dart:collection';
 import 'dart:_internal';
 import 'dart:isolate';
@@ skipping 105 matching lines @@
 bool _isWindows = false;
 
 // Logging from builtin.dart is prefixed with a '*'.
 String _logId = (Isolate.current.hashCode % 0x100000).toRadixString(16);
 _log(msg) {
   _print("* $_logId $msg");
 }
 
 // A class wrapping the load error message in an Error object.
 class _LoadError extends Error {
+  final _LoadRequest request;
   final String message;
-  final String uri;
-  _LoadError(this.uri, this.message);
+  _LoadError(this.request, this.message);
 
-  String toString() => 'Load Error for "$uri": $message';
+  String toString() {
+    var context = request._context;
+    if (context == null || context is! String) {
+      return 'Could not load "${request._uri}": $message';
+    } else {
+      return 'Could not import "${request._uri}" from "$context": $message';
+    }
+  }
 }
 
 // Class collecting all of the information about a particular load request.
 class _LoadRequest {
   final int _id = _reqId++;
   final int _tag;
   final String _uri;
   final Uri _resourceUri;
   final _context;
 
@@ skipping 224 matching lines @@
         c.complete(dataOrError);
       } else {
         // TODO: Currently a compilation error while loading the script is
         // fatal for the isolate. _loadScriptCallback() does not return and
         // the number of requests remains out of sync.
         _loadScriptCallback(req._tag, req._uri, req._context, dataOrError);
       }
       _finishLoadRequest(req);
     } else {
       assert(dataOrError is String);
-      var error = new _LoadError(req._uri, dataOrError.toString());
+      var error = new _LoadError(req, dataOrError.toString());
       _asyncLoadError(req, error, null);
     }
   } catch(e, s) {
     // Wrap inside a _LoadError unless we are already propagating a
     // previous _LoadError.
-    var error = (e is _LoadError) ? e : new _LoadError(req._uri, e.toString());
+    var error = (e is _LoadError) ? e : new _LoadError(req, e.toString());
     assert(req != null);
     _asyncLoadError(req, error, s);
   }
 }
 
 
 void _startLoadRequest(int tag, String uri, Uri resourceUri, context) {
   if (_dataPort == null) {
     if (_traceLoading) {
       _log("Initializing load port.");
@@ skipping 177 matching lines @@
 }
 
 
 _loadDataFromLoadPort(int tag, String uri, Uri resourceUri, context) {
   try {
     _startLoadRequest(tag, uri, resourceUri, context);
   } catch (e, s) {
     if (_traceLoading) {
       _log("Exception when communicating with service isolate: $e");
     }
+    // Register a dummy load request so we can fail to load it.
+    var req = new _LoadRequest(tag, uri, resourceUri, context);
+
     // Wrap inside a _LoadError unless we are already propagating a previously
     // seen _LoadError.
-    var error = (e is _LoadError) ? e : new _LoadError(uri, e.toString());
-    // Register a dummy load request and fail to load it.
-    var req = new _LoadRequest(tag, uri, resourceUri, context);
+    var error = (e is _LoadError) ? e : new _LoadError(req, e.toString());
     _asyncLoadError(req, error, s);
   }
 }
 
 
 // Loading a package URI needs to first map the package name to a loadable
 // URI.
 _loadPackage(int tag, String uri, Uri resourceUri, context) {
   if (_packagesReady) {
     var resolvedUri;
     try {
       resolvedUri = _resolvePackageUri(resourceUri);
     } catch (e, s) {
       if (_traceLoading) {
         _log("Exception ($e) when resolving package URI: $resourceUri");
       }
+      // Register a dummy load request so we can fail to load it.
+      var req = new _LoadRequest(tag, uri, resourceUri, context);
+
       // Wrap inside a _LoadError unless we are already propagating a previously
       // seen _LoadError.
-      var error = (e is _LoadError) ? e : new _LoadError(uri, e.toString());
-      // Register a dummy load request and fail to load it.
-      var req = new _LoadRequest(tag, uri, resourceUri, context);
+      var error = (e is _LoadError) ? e : new _LoadError(req, e.toString());
       _asyncLoadError(req, error, s);
     }
     _loadData(tag, uri, resolvedUri, context);
   } else {
     if (_pendingPackageLoads.isEmpty) {
       // Package resolution has not been setup yet, and this is the first
       // request for package resolution & loading.
       _requestPackagesMap();
     }
     // Register the action of loading this package once the package resolution
@@ skipping 263 matching lines @@
 
 // Register callbacks and hooks with the rest of the core libraries.
 _setupHooks() {
   _setupCompleted = true;
   VMLibraryHooks.resourceReadAsBytes = _resourceReadAsBytes;
 
   VMLibraryHooks.packageRootUriFuture = _getPackageRootFuture;
   VMLibraryHooks.packageConfigUriFuture = _getPackageConfigFuture;
   VMLibraryHooks.resolvePackageUriFuture = _resolvePackageUriFuture;
 }