Improve error messages in MirrorSystem.findLibrary.

runtime/lib/mirrors_patch.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.
 
 import "dart:_internal" as _symbol_dev;
 
 /**
  * Returns a [MirrorSystem] for the current isolate.
  */
 patch MirrorSystem currentMirrorSystem() {
@@ skipping 18 matching lines @@
  */
 patch ClassMirror reflectClass(Type key) {
   return _Mirrors.reflectClass(key);
 }
 
 patch TypeMirror reflectType(Type key) {
   return _Mirrors.reflectType(key);
 }
 
 patch class MirrorSystem {
+  /* patch */ LibraryMirror findLibrary(Symbol libraryName) {
+    var candidates =
+        libraries.values.where((lib) => lib.simpleName == libraryName);
+    if (candidates.length == 1) {
+      return candidates.single;
+    }
+    if (candidates.length > 1) {
+      var uris = candidates.map((lib) => lib.uri.toString()).toList();
+      throw new Exception("There are multiple libraries named "
+                          "'${getName(libraryName)}': $uris");
+    }
+    throw new Exception("There is no library named '${getName(libraryName)}'");
+  }
+
   /* patch */ static String getName(Symbol symbol) {
     String string = _symbol_dev.Symbol.getName(symbol);
 
     // get:foo -> foo
     // set:foo -> foo=
     // get:_foo@xxx -> _foo
     // set:_foo@xxx -> _foo=
     // Class._constructor@xxx -> Class._constructor
     // _Class@xxx._constructor@xxx -> _Class._constructor
     // lib._S@xxx with lib._M1@xxx, lib._M2@xxx -> lib._S with lib._M1, lib._M2
@@ skipping 42 matching lines @@
 // TODO(rmacnak): Eliminate this class.
 class MirroredCompilationError {
   final String message;
 
   MirroredCompilationError(this.message);
 
   String toString() {
     return "Compile-time error during mirrored execution: <$message>";
   }
 }