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);
-    // 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);
-  }
-
   /// Returns every [ImportElement] that imports [element] into [library].
   Iterable<ImportElement> _getImports(Element element, LibraryElement library) {
     if (element.isClassMember) {
       element = element.enclosingClass;
     }
     if (element.isAccessor) {
       element = (element as AccessorElement).abstractField;
     }
     return library.getImportsFor(element);
   }
@@ skipping 263 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);

[sra1, 2017/08/16 01:09:22]

Presumably this is actually a List.

[Siggi Cherem (dart-lang), 2017/08/16 01:18:20]

Ack - I'm using the declared type for now. I feel we would have to update that
in the element model too if we wanted to use it as List here.

+      bool isExplicitlyDeferred =

[sra1, 2017/08/16 01:09:22]

This can still be a function, just taking the iterable instead.
That gives somewhere nicer for the comments to live and avoids needing to
understand !x && y

[Siggi Cherem (dart-lang), 2017/08/16 01:18:20]

Done. Moved it was it used to be, but made the list of imports an argument
instead.

+          // If the element is not imported explicitly, it is implicitly
+          // imported not deferred.
+          !imports.isEmpty &&
+              // An element could potentially be loaded by several imports. If
+              // all of them are explicitly deferred, we say the element is
+              // explicitly deferred.
+              // TODO(sigurdm): We might want to give a warning if the imports
+              // do not agree.
+              imports.every((i) => i.isDeferred);
+      if (isExplicitlyDeferred) {
+        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';
 }