Make 'where' lazy.

sdk/lib/_internal/compiler/implementation/dart_backend/backend.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.
 
 part of dart_backend;
 
 // TODO(ahe): This class is simply wrong.  This backend should use
 // elements when it can, not AST nodes.  Perhaps a [Map<Element,
 // TreeElements>] is what is needed.
 class ElementAst {
@@ skipping 432 matching lines @@
     final unparser = new EmitterUnparser(renames);
     emitCode(unparser, imports, topLevelNodes, memberNodes);
     compiler.assembledCode = unparser.result;
 
     // Output verbose info about size ratio of resulting bundle to all
     // referenced non-platform sources.
     logResultBundleSizeInfo(topLevelElements);
   }
 
   void logResultBundleSizeInfo(Set<Element> topLevelElements) {
-    Collection<LibraryElement> referencedLibraries =
+    Iterable<LibraryElement> referencedLibraries =
         compiler.libraries.values.where(isUserLibrary);
     // Sum total size of scripts in each referenced library.
     int nonPlatformSize = 0;
     for (LibraryElement lib in referencedLibraries) {
       for (CompilationUnitElement compilationUnit in lib.compilationUnits) {
         nonPlatformSize += compilationUnit.script.text.length;
       }
     }
     int percentage = compiler.assembledCode.length * 100 ~/ nonPlatformSize;
     log('Total used non-platform files size: ${nonPlatformSize} bytes, '
@@ skipping 91 matching lines @@
 }
 
 compareElements(e0, e1) {
   int result = compareBy((e) => e.getLibrary().uri.toString())(e0, e1);
   if (result != 0) return result;
   return compareBy((e) => e.position().charOffset)(e0, e1);
 }
 
 List<Element> sortElements(Collection<Element> elements) =>
     sorted(elements, compareElements);