Better dependency tracking

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

+library dart2js.dependency_information;
+
+import 'dart2jslib.dart';
+import 'elements/elements.dart';
+import 'tree/tree.dart';
+
+
+abstract class DependencyInformation {
+  factory DependencyInformation(Enqueuer enqueuer, bool dumpInfoEnabled) {
+    if (dumpInfoEnabled) {
+      return new _DependencyInformation(enqueuer);
+    } else {
+      return new _EmptyDependencyInformation();
+    }
+  }
+
+  void elementUsesSelector(Element element, Selector selector) {}

[sra1, 2014/07/30 21:43:48]

abstract methods are declared without a body:

void elementUsesSelector(Element element, Selector selector);

[Ty Overby (Google), 2014/08/01 16:39:37]

This class is gone.  
Done.

+  Iterable<Element> getRetaining(Element element, Compiler compiler) {}
+}
+
+class _EmptyDependencyInformation implements DependencyInformation {
+  _EmptyDependencyInformation();
+
+  void elementUsesSelector(Element element, Selector selector) {}
+  Iterable<Element> getRetaining(Element element, Compiler compiler) {
+    return const <Element>[];
+  }
+}
+
+
+class _DependencyInformation implements DependencyInformation {
+  _DependencyInformation(Enqueuer enqueuer);
+
+  /// An [Element] uses [Selector]s
+  final Map<Element, Set<Selector>> selectorsFromElement = {};
+
+  /// Returns a group of [Element]s that [element] is retaining.
+  Iterable<Element> getRetaining(Element element, Compiler compiler) {
+    if (!selectorsFromElement.containsKey(element)) {
+      return [];
+    } else {
+      return selectorsFromElement[element].expand(
+          (s) => compiler.world.allFunctions.filter(s));
+    }
+  }
+
+  // The Element may or may not actually be emitted.
+  void elementUsesSelector(Element element, Selector selector) {
+    selectorsFromElement
+        .putIfAbsent(element, () => new Set<Selector>())
+        .add(selector);
+  }
+}