Change MirrorSystem.libraries to Map<Uri, String>

sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart

Index: sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart
diff --git a/sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart b/sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart
index dd8c70fdbaacd0c2c1d29af60fce3ec72b454f24..de1cedad5dd3813b662195ec79c8a23786662ed5 100644
--- a/sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart
+++ b/sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart
@@ -17,8 +17,16 @@ abstract class MirrorSystem {
   /**
    * Returns an unmodifiable map of all libraries in this mirror system.
    */
-  // TODO(johnniwinther): Change to Map<Uri, LibraryMirror>.
-  Map<String, LibraryMirror> get libraries;
+  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(
+        (library) => library.simpleName == libraryName);
+  }
 
   /**
    * A mirror on the [:dynamic:] type.