Mirrors overhaul.

dart/sdk/lib/_internal/compiler/implementation/mirrors/mirrors.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 mirrors;
 
 /**
  * The main interface for the whole mirror system.
  */
 abstract class MirrorSystem {
   /**
    * Returns an unmodifiable map of all libraries in this mirror system.
    */
   Map<Uri, LibraryMirror> get libraries;
 
   /**
    * Returns an iterable of all libraries in the mirror system whose library
    * name is [libraryName].
    */
-  Iterable<LibraryMirror> findLibrary(String libraryName) {
-    return libraries.values.where(
+  LibraryMirror findLibrary(String libraryName) {
+    return libraries.values.singleWhere(
         (library) => library.simpleName == libraryName);
   }
 
   /**
    * A mirror on the [:dynamic:] type.
    */
   TypeMirror get dynamicType;
 
   /**
    * A mirror on the [:void:] type.
@@ skipping 657 matching lines @@
   /**
    * Returns the URI where the source originated.
    */
   Uri get sourceUri;
 
   /**
    * Returns the text of this source.
    */
   String get sourceText;
 }