call _getImports only once: on a large app this was about 5s extra.

pkg/compiler/lib/src/deferred_load.dart

 // Copyright (c) 2014, 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 deferred_load;
 
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'constants/expressions.dart' show ConstantExpression;
@@ skipping 243 matching lines @@
 
   void registerConstantDeferredUse(
       DeferredConstantValue constant, PrefixElement prefix) {
     OutputUnit outputUnit = new OutputUnit();
     outputUnit.imports.add(new _DeclaredDeferredImport(prefix.deferredImport));
 
     // Check to see if there is already a canonical output unit registered.
     _constantToOutputUnit[constant] = _getCanonicalUnit(outputUnit);
   }
 
-  /// Answers whether [element] is explicitly deferred when referred to from
-  /// [library].
-  bool _isExplicitlyDeferred(Element element, LibraryElement library) {
-    Iterable<ImportElement> imports = _getImports(element, library);
+  /// Given [imports] that refer to an element from a library, determine whether
+  /// the element is explicitly deferred.
+  static bool _isExplicitlyDeferred(Iterable<ImportElement> imports) {
     // If the element is not imported explicitly, it is implicitly imported
     // not deferred.
     if (imports.isEmpty) return false;
     // An element could potentially be loaded by several imports. If all of them
     // is explicitly deferred, we say the element is explicitly deferred.
     // TODO(sigurdm): We might want to give a warning if the imports do not
     // agree.
     return imports.every((ImportElement import) => import.isDeferred);
   }
 
@@ skipping 273 matching lines @@
       elements.add(element);
 
       // This call can modify [dependentElements] and [dependentConstants].
       _collectAllElementsAndConstantsResolvedFrom(
           element, dependentElements, dependentConstants, isMirrorUsage);
 
       library = element.library;
     }
 
     for (Element dependency in dependentElements) {
-      if (_isExplicitlyDeferred(dependency, library)) {
-        for (ImportElement deferredImport in _getImports(dependency, library)) {
+      Iterable<ImportElement> imports = _getImports(dependency, library);
+      if (_isExplicitlyDeferred(imports)) {
+        for (ImportElement deferredImport in imports) {
           _mapDependencies(
               element: dependency,
               import: new _DeclaredDeferredImport(deferredImport));
         }
       } else {
         _mapDependencies(element: dependency, import: import);
       }
     }
 
     for (ConstantValue dependency in dependentConstants) {
@@ skipping 545 matching lines @@
 
   bool operator ==(other) {
     if (other is! _DeclaredDeferredImport) return false;
     return declaration == other.declaration;
   }
 
   int get hashCode => declaration.hashCode * 17;
 
   String toString() => '$declaration';
 }