Improve error messages in MirrorSystem.findLibrary.

sdk/lib/mirrors/mirrors.dart

 // Copyright (c) 2013, 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.
 
 // For the purposes of the mirrors library, we adopt a naming
 // convention with respect to getters and setters.  Specifically, for
 // some variable or field...
 //
 //   var myField;
 //
@@ skipping 61 matching lines @@
    * Returns an immutable map from URIs to mirrors for all
    * libraries known to this mirror system.
    */
   Map<Uri, LibraryMirror> get libraries;
 
   /**
    * Returns the unique library named [libraryName] if it exists.
    *
    * If no unique library exists, an error is thrown.
    */
-  LibraryMirror findLibrary(Symbol libraryName) {
-    return libraries.values.singleWhere(
-        (library) => library.simpleName == libraryName);
-  }
+  LibraryMirror findLibrary(Symbol libraryName);
 
   /**
    * A mirror on the isolate associated with this [MirrorSystem].
    * This may be null if this mirror system is not running.
    */
   IsolateMirror get isolate;
 
   /**
    * A mirror on the [:dynamic:] type.
    */
@@ skipping 1123 matching lines @@
    *
    * When used as metadata on an import of "dart:mirrors", this metadata does
    * not apply to the library in which the annotation is used, but instead
    * applies to the other libraries (all libraries if "*" is used).
    */
   final override;
 
   const MirrorsUsed(
       {this.symbols, this.targets, this.metaTargets, this.override});
 }