Better dependency tracking

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

-library dart2js.compilation_info;
-
-import 'dart2jslib.dart';
-import 'elements/elements.dart';
-import 'tree/tree.dart';
-
-
-abstract class CompilationInformation {
-  factory CompilationInformation(Enqueuer enqueuer, bool dumpInfoEnabled) {
-    if (dumpInfoEnabled) {
-      return new _CompilationInformation(enqueuer);
-    } else {
-      return new _EmptyCompilationInformation();
-    }
-  }
-
-  Map<Element, Set<Element>> get enqueuesMap;
-  Map<Element, Set<Element>> get addsToWorkListMap;
-
-  void enqueues(Element function, Element source) {}
-  void addsToWorkList(Element context, Element element) {}
-  void registerCallSite(TreeElements context, Send node) {}
-}
-
-class _EmptyCompilationInformation implements CompilationInformation {
-  _EmptyCompilationInformation();
-  Map<Element, Set<Element>> get enqueuesMap => <Element, Set<Element>>{};
-  Map<Element, Set<Element>> get addsToWorkListMap => <Element, Set<Element>>{};
-
-  void enqueues(Element function, Element source) {}
-  void addsToWorkList(Element context, Element element) {}
-  void registerCallSite(TreeElements context, Send node) {}
-}
-
-
-class _CompilationInformation implements CompilationInformation {
-  final String prefix;
-
-  final Map<Element, Set<Element>> enqueuesMap = {};
-  final Map<Element, Set<Element>> addsToWorkListMap = {};
-
-  _CompilationInformation(Enqueuer enqueuer)
-    : prefix = enqueuer.isResolutionQueue ? 'resolution' : 'codegen';
-
-  Set<CallSite> callSites = new Set<CallSite>();
-
-  enqueues(Element function, Element source) {
-    enqueuesMap.putIfAbsent(function, () => new Set())
-    .add(source);
-  }
-
-  addsToWorkList(Element context, Element element) {
-    addsToWorkListMap.putIfAbsent(context, () => new Set())
-    .add(element);
-  }
-
-  registerCallSite(TreeElements context, Send node) {
-    callSites.add(new CallSite(context, node));
-  }
-}
-
-class CallSite {
-  final TreeElements context;
-  final Send node;
-  CallSite(this.context, this.node);
-}