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

pkg/compiler/lib/src/deferred_load.dart

Index: pkg/compiler/lib/src/deferred_load.dart
diff --git a/pkg/compiler/lib/src/deferred_load.dart b/pkg/compiler/lib/src/deferred_load.dart
index fe77fd4bcad0d127e722fd624772f0631cb09b2a..43b3ebb684c052ad2622646b735946ec6cd39372 100644
--- a/pkg/compiler/lib/src/deferred_load.dart
+++ b/pkg/compiler/lib/src/deferred_load.dart
@@ -261,20 +261,6 @@ class DeferredLoadTask extends CompilerTask {
     _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) {
@@ -558,8 +544,19 @@ class DeferredLoadTask extends CompilerTask {
     }
 
     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));